You will need certain items and pokemon to gain access to these secret pokemon. Look below.

Unlockable:	How to unlock:
Regigigas lv 70	You will need to migrate Regice, Regirock, and Registeel into your current Diamon/Pearl game. Defeat the Elite Four and complete the Sinnoh Pokedex. Go to the temple in Canalave City with the three Regi's in your party. Slide and smash through the temple until you reach the final floor, where a statue is standing in the middle if the room. It is difficult to get to, as you must slide round on the ice for a while, taking the right combination of slide paths. When you finally reach it, save the game. Reboot and
Giratina lv 70	You will need to beat the Elite Four and compleye the Sinnoh Pokedex again. This time, the route above lake Valor has another exit about half way up, previously, where trees were. Go down this path. Hint: you will need Defog. At the end will be a misty pond with a bridge. Ignore the bridge, it goes nowhere. Go round to the cave entrance. Choose any combination of tunnels inside the cave, but after you pass three pillars, Giratina will appear in the next room. Save the game. It is extremely hard to catch. Dusk balls are your best bet.
Cresselia lv 50	Defeat Elite Four, complete Sinnohdex. Go to Canalave City and find Sailor Elrich's house West of the city. Talk to the little boy in the bed and the mother beside him. Go outside and talk to the sailor by the boat, this is Elrich. He will give you an item that summons the legendary pokemon Cresselia. Go to New Moon Island and awaken the pokemon there. It will now appear on the marking map. It will run away from you if you fly to it, so, use your bike, or walk to it. It will appear at random. Weaken it until it into critical health (red bar). This may take several attempts and you may need to encounter Cresselia a few times before you can actually catch it. Use a quick ball once it has hardly any health. Hint: Save the game just before you know you are going to meet it; this is just in case you KO it. It won't come back. I used a masterball for this pokemon, as I found it the trickiest to catch after it will escape from you in it's first turn. You could use Mean Look to make it stay, but it is a powerful pokemon and once your Mean Look pokemon faints, Cresselia is free to escape again.
Rotom lv 15	Must beat the Eterna Gym Leader. Go back to Eterna forest and find the mansion to the north of the woods. Use cut to gain access to the garden and enter the house. Go to the top floor. enter northern room and second from the left end of the corridor is a room with a TV in it.
Arceus lv 80	Apologies, this requires a Nintendo download or an Action Replay (AR) cheat code. Arcues can only be summoned at Spear Pillar with the use of Azure Flute, obtained through Mystery Gift.This only works of you have either of the two cheats mentioned above.
Darkrai lv 70	This also requires an AR cheat code. Darkrai resides on Full Moon Island and is the pokemon opposite of Cresselia.
Heatran lv 70	Beat the Elite Four and gain access to the Battle Area etc. On this island, there will be a volcano to the north. Find your way into this volcano and be accompanied by a guy with red hair. [Sorry, I forget his name] He will be with you until you find the magma stone to the very north of the volcan cave. He will take the magma stone out with him. Exit the volcano and fly to the Survival area. Somewhere, here, you will find his house, along with his grandad. Little red riding hood will replace the magma stone back into Stark Mountain and you should follow him. Go back to where the magma stone was located before and here you will find Heatran. It will either be male or female, though cannot breed. Save game before catching. Heatran is tricky to get to a low enough health and quite tricky to catch.
Porygon-Z	Porygon-Z is the next stage of Porygon, that follows on from Porygon-2. You can obtain this pokemon by trading it with the Dubious Disk.
Dialga lv 47	After rescuing the three legendary pokemon, Azelf, Uxie and Mesprit, you will go through an event that leads you to Dialga. You must scale Mount Coronet from the Hearthrome entrance. At th top you will team up with your Rival [mine is called Alex :) ] and you and him will battle together against two of the Team Galactic Commanders. After that, you will battle Cirrus and he will summon Dialga with the chain to stop you. Battle/catch Dialga, but remember to save the game before the encounter.
Palkia lv 47	The same as for encountering Dialga, but with Pearl version. Palkia will only appear once in the Pearl version, likewise with Dialga in the Diamon version.
Uxie lv 50	Uxie will be at Lake Acuity. Go to the cave in the middle of the lake and save the game. Reboot and
Azelf lv 50	Azelf will be at Lake Valor. Go to the cave in the middle of the lake and save the game. Reboot and
Mesprit lv 50	Mesprit will be at Lake Verity. There is no need to save the game here. Mesprit will run away and expects you to play games with it. Use the same tactic for catching Cresselia, by using the marker map. It is the hardest to catch and the most difficult to get down to low health.
Galade	You will need to evolve a male Kirlia with a Dawn Stone. It is the mle counterpart to Gardevoir.
Leafeon	Leafeon can be obtained by leveling up an Eevee by the moss covered rock by the entrance to Eterna Forest. You can obtain Eevee from Bebe the storage geek in Hearthrome, or from Pokemon masion. [Talk to Mt Backlot. Answer all his questions with
Glaceon	Obtained in the same was as Leafeon. This time however, you must level up the Eevee by the Ice covered rock just south of Canalave.
Togekiss	Migrate a Togepi or Togetic from Leaf-Green or Fire-Red games. Evolve the Togepi by making it happy. Hint: Use the poketch app to keep an eye on its happiness. Use Poffins, Stats enhancers, Massages and keep it in your party to make it happier quicker.] Once you have Togetic, use a Shiny stone.
Riolu lv 1	Go to Iron Island and descend until you meet with Riley, the strapping young man in the smart blue suit. Sexy :] Let him accompany you through the cave until you encounter two Galactics. Restore peace to Iron Island by ridding the cave of these two alien-folk and he will give a departing gift of an egg. Walk round with this egg [Hint: A pokemon with the flame-body ability, E.g, Slugma, will cut this hatch time in half.] and it will hatch eventually. You will get Riolu which is the pre-eveolved form of Lucario.