Hey i think i'm trying to do somethign just like you. But what your trying to do. So you hit the button, the arduino sends serially, an A into the computer. Then if the other button is pressed it'll do a D instead.
If thats what your trying. It wouldn't be too hard to makeup a sketch on the arduino to send in one of those 2 letters when a button is pressed. Just a digital button tutorial doubled with a serial.print
as for the comptuer side of things. a couple of older useres of this froum told me to look up "gobetwino".
Its a nifty program that will let you do stuff based on a serial signal in.
Heres the website. Gadgets og teknologi | Dansk Tech Blog - Mikmo
I'm still messing with getting it to work though, havent gotten to play with it yet. But it might be just what you need.