Is it crashing or is it just going to normal stage builder?
If it's going to normal stage builder, that has happened to me before, you should wait like 30 seconds after booting up the game before you navigate to stage builder. It's something to do with Brawl preparing everything I think.
If it's crashing I need specifics.
Regardless of what it's doing,

make sure your SD card is UNLOCKED (so the lock nob is up)