After you have the super missiles, you can fight Kraid. Go open the green dorr, and return to the elavtor that leads down to Norfair. Once in the elavator room, use the super missiles to bloww open the rooms right wall. Hope that helped.
Okay, time somebody took pity on the guy. Shockna, if you're stuck where we think you're stuck, you're in a room with a long collapsing platform you can't get across. What you need to do is hold down the Run button. (If you're using a SNES, the default is B; I like to map it to R by analogy with Mega Man Zero. If you're using an emulator, you'll have to look up what key represents B.)
That platform is called the "n00b bridge" because countless n00bs don't know about the Run button and get stuck there. I know I did.