You can use your keyboard, the middle row maps to the white keys and the upper row maps to black keys. Works better than a mouse for tempo, IMO.
I keep getting confused because I'm so used to the old-style tracker keys (I keep hitting J for Bb and getting B, which isn't helping my C minor improv). Also one octave isn't great, lag is killing the rhythm, and it keeps not waiting for me to finish.
Neat idea though.
(I'm curious to know whether the training set for the version shown in the demo is the same as for the live web version, as the demo sounded way better than anything I'm getting out of it - though obviously they would have cherrypicked the results for the video.)