Sega Genesis / Mega Drive Emulator DGEN v1.70 Lite - PSP Slim Support » PSP Slim Hacks
Go there download the 1.7 lite version of dgen
then when you unzip it you'll have two files an eboot and a text, the text is useless so forget it
Connect your psp using a usb and go to the psp/game folder
so X:PSP/GAME/
Make a new folder called DGEN
so it will look like
X:PSP/GAME/DGEN
Place the eboot in here, then make a new folder next to it, and name that folder ROMS
So it will look like this
X: PSP/GAME/DGEN/ROMS
Place your roms in that folder and exit, go to game on your psp, and loook for dgen
Once it starts, you need to find where the games are stored, its like a computer, it won't start unless you tell it what to do, so find the folder that says roms and push O, then select your game and you should be on your way