If you check the manual or especially the quick reference guide it has some info about mapping, but here's the short version:
* For tremolo picking, make sure tremolo is turned on (check the front page of the UI, or the "Articulations" page). Then, check the Articulations page to see what velocity range Tremolo is mapped to. By default, it's velocity = 1, but you can change it to whatever you want.
* Pick scrapes and neck slides are in the patch labeled Shreddage 2 FX.