The only thing that is not very obvious is that if you don't wan't to hit a drum to get the exact velocity in the editor you can hover over a note and use the + and - keys to adjust the volume.

If you click on a slot on one of the lanes it will also use the last velocity when you are editing.

Finally wrote a quick web app to help practice timing and velocity on #drums. If anyone wants to try it out just go to codapagoda.com, plug in a #midi drum set, and hit some pads.