Darkrai: To get it, on a random day (screw around with your calendar) then get the Mystery Gift. Get the Member Pass from the guy with a green suit in the Pokemart. Then, go to Canclave City, to the Hotel Inn, and go inside. A dead dude will be in there. He'll make you go to sleep on the bed. You will dream that you are on Newmoon Island. Go to the woods, where Darkrai is. Either kill it or catch it, and you will be back to reallity. You will now be able to go to Newmoon Island anytime.

Arceus: On a random day, (again screw your calendar) then get the Mystery Gift. Go to the Pokemart, and get the Azure Flute. Then, go to Spear Piller, the Azure Flute will start making noise, blow into it. Invisible stairs will appear. This is the Hall of Origin. Arceus will appear out of nowhere. You need to use Wrap before you use a Master Ball. Then, go to Oregburgh Mine, there will be this Hiker dude there, talk to him. He will give you a Flame Plate. Give Arceus a Plate to hold, and Arceus will become that type of whatever Plate it's holding. Go to the Canclave Library, on the second floor. The same Hiker guy will be there. He'll talk, and says he wrote a book about the Plates. Then, go to Celestic Town. Go in the cave. Make sure Dialga, Palkia, Giratina, and Arceus is in your party. Click on the painting, and Cynthia will go there. The three things on the painting are Dialga, Palkia, and Giratina. The sphere is Arceus. Ok moving on.

Shaymin: Again on a random day, (you know what to do.) then get the Mystery Gift. Go to the Pokemart and talk to the guy in the green suit. He'll give you the Oak's Letter. Go to Victory Road, but where the fat guy was, and you need to go through to Route 224. Go to the rock, and Oak will be there. He'll ask you who you thank. Say ME. Seabreak Path will appear, and you'll see Shaymin. Chase after it, until you get to Flower Paradise. Battle Shaymin, and catch it.

Sky Forme: Make sure Shaymin is in your party. Go to Floamora Town, and talk to the blonde-haired lady in the middle of the town. She will give you a Gracidea. It is a Key Item though, so if you put Shaymin in a PC box, it will become regular forme. And at night, it will become Land Forme.

There are two other things the green suited guy will give you, the Secret Key will be needed with Rotom in your party. Go to Eterna City, to the Galactic Building. On the first floor, next to the yellow machine, is a little space where the Secret Key will unlock a little passage where there will be a lawn mower, a microwave, a refridgerater, a washing machine, and a rotary fan. You should be able to figure out what to do.
The second thing he'll give you is a Manaphy Egg. It takes A LONG time to hatch, but that is how you get Manaphy added to your Pokedex. I hope these helped!