How do you unlock the rabbits.?

Are you talking about mini-game rabbits? If you are, I only know one for Luigi, 4 for Mario, and (I think) all for Yoshi. For Luigi, get 150 stars to get a cannon outside the castle. Shoot yourself to the roof. To get Mario's, there are two outside the castle. One is in the moat, the other might be near the waterfall somewhere. The other two are inside the basement near the painting to Lethal Lava Land. Go on the left to trap one of them. The other one goes in a certain route around 4 walls. Memorize it to catch it. For Yoshi, there is only one bunny in the basement of where I said Mario's are. 5 of them are in the front. 3 are near the entrance to the castle, 2 are in flowers somewhere, and that's it. If your talking about Shining Rabbits, I only know 2. Both are in the basement, but you might get only one chance to get them. You have to be Mario or Yoshi. They are in the same place as their original bunny. Only thing is that the original bunny is replaced with the Shining Bunny.

If you mean by the Normal Rabbits, I think you have to beat the first Bowser And then the rabbits will appear outside and around the castle, But If You mean By the Glowing Rabbits, I think They Either Appear After You beat Some Certain Levels Or they may Just Appear Randomly.

It's me again. I found two more bunnies for Luigi. One is in the area that looks like a mini-game from Mario-Party Advance for GBA(Grabbit). If you don't know what I'm talking about, the bunny is in the area of the first bunny in the whole game. The other one is in the room with the Mario painting in there.