It appears that despite what the controls page indicates (yo, where's controller config?) the controls are Z, X, C, A for A, B, C, D respectively.

Instruction is Misrepresentation

maybe

Z,X,C,A / A,B,C,D

I'm guessing it's that's the problem with found in the Humble Store versions of these games, which gives you a front-end with no rebindable keys whatsoever. Seriously, DotEmu, can you be more inconsiderate for your userbase? How is it that only a small handful these games like the Metal Slug games and Twinkle Star Sprites gotten better emulated ports and the rest put together with a sub-par emulator? In any case, assuming it's the Humble Bundle version being brought into GOG, you should be able to take the ROMs you got with the game and use them on a superior emulator like MAME or Final Burn Alpha with more options and better aspect-ratio correction than the default front-end.

Wow, thanks for this. I just wasted hours trying to get it working.