In sunvox, all you have to do is to select your midi controller device in Preferences -> MIDI. However, before that, you need the alsa midi drivers. I had to compile them myself since they don't come with the n900 kernel packages. I'm using 2.6.28.10-power49 (power kernel).
I just uploaded them to: http://202.65.223.218/~pigeon/power49-midi-modules.tgz I also have a shell script that sudo insmod the modules in the right order.