Mysterie solved :).
The Adafruit MIDIsense board has a ATmega168 on it, and no usb. So if I want to use Arduino with it, I simply choose "sketch | verify/compile" and then "-> upload to board". It gives an error, because there's no serial link ofcourse, but it creates a folder called "applet" in de projectfolder, with the [projectname].hex file in it.
Then with avrdude and avrisp mkII I can upload this hex file into the board.
Ofcourse this way i've got no bootloader, but who cares..
Today I got my avrisp mkII, which is nice.. but it only had 6 pin header connectpr, so I demolished the 10 pin header, to become a 6 pin one, and then rewired it.. lucky that I like to solder a lot.. 