What do I do with MAME roms?

im familiar with snes where you just dl a rom and drop into rom folder, but with mame lets say i want to play xmen vs streetfighter, on a certain site theres a 22 mb version and a 1.3 mb version, which do i dl? also confused when people say you have to have the right rom set? do i have to convert something? on Mameforall 0.0.3 when i put the zip file into rom folder the emu says no roms found. is it because the game isnt compatible? thanks.

I would expect so. I use EEMAME on my phone. It basically runs the smaller MAME ROMs. All you have to do is Place the ROM, still in a zipped folder into the "ROMs" folder. If it doesn't work, check your version of MAME, and have a look around http://www.mame.net/ to see if there is any info on compatible ROMs and MAME versions.