There are a few steps to unlocking the Regis. Read them carefully.
Step 1: Go to Pacificlog Town.
Step 2: Use Surf to get into the current west of Pacificlog Town.
Note: You will end up on land a few times.
Step 3: If you stayed around the bottom of the current ( not the very bottom), you should end up on water with no current. It is supposed to be shaped like a +.
Step 4: Within that space of water, thre should be a smaller +, a darker shade. Go on to the smaller + and use dive.
Step 5: Underwater, go down and you should see some dots. This is braille. It says "Go up here". So go up.
Step 6: You should be in a place called "Sealed Chamber". There are rocks in this room. They are just the braille alphabet from A to Z. Ignore thse and go to the ery top of the room.
Step 7: There should be more braille at the top of the room. It reads "Use dig here." Use dig and the wall there will crumble and create an entrance.
Step 8: Go into the second room. Again, there will be more rocks with more braille. I think it is supposed to describe the history of the Regis. Go to the top of the room.
Step 9: At the top of the room , there will be (surprise, surprise!) more braille. This braille reads "First comes Wailord and last comes Relicanth." In simpler words, this means to get a Wailord (evolved form of Wailmer) and put it in the first spot of your party and put Relicanth at the bottom of your party.
Step 10: When you do this, there should be some rumbling and the sound of three doors opening. Your trainer should say "It sounds af if some doors opened far away. Then you have unlocked the Regis!
Regirock is in the bottom of the desert.There also some braille in the cave that Regirock is in. It says "Left, left, down, down, then use Rock Smash. Go to the next room and prepare to fight Regirock.
Regice is north of Dewford Town. When you enter the cave, go to the top of the room. Then it tells you to do one lap around the cave while touching the wall. The wall should crumble. Go to the next room and prepare to fight Regice.
Note: There should a few mini islands filled with trees nearby Regice's cave.
Now for the most Powerful Regi. Registeel. Go to the route right of Fortree City. Be fore you enter a grass maze, it should start to rain. Get to the end of the grass maze and it should stop raining. Go down for a bit and go up some steps. When you are near Registeel's cave, it should start to rain again. When you are iin Registeel's cave, go to the top of the room. There is yet again more braille to decode. The braille reads "With new time, hope and love, aim for the sky in the middle." This basically means to go to the very center of the room and use fly. The wall should crumble. Get in there and prepare to fight Registeel.
Note: All the Regi's are difficult to capture with Registeel being the most difficult. Relicanth is rare and really hard to find. You can only find it underwater in the seaweed.