An Intel-based MacBook has almost the same hardware as a normal PC: an Intel Core 2 Duo processor, Intel chipset, Intel graphics chip, etc. There's no problem that has to do with 32- or 64-bitness. The 32-bit as well as the 64-bit version of Ubuntu should both work on a MacBook.
I don't have a MacBook myself, but you can find more info here:
https://help.ubuntu.com/community/MacBook