You have to "remap" mame's inputs. Four ways
1) "TAB" method: check out
mame.net FAQ easyemu specific page, (
easyemu index)
2) "ctrlr ini" files method: check out
easyemu ctrlr ini page& docs/ctrlr.txt file that comes with mame.
3) "unused joystick" method
Uninstall your current joystick, install a different joystick you won't use, then install the joystick you want to use. Do not uninstall the unused joystick.

4) "Compile your own" method
edit the mame source and recompile your own binary.

Alright, only the first two are the common solutions. Personally, I like #2 better, since it is designed to work between mame versions. #1 is easier to understand, though. *shrug*