I stopped using Sony Acid years ago. But get the manual and read the parts about MIDI tracks and VST instruments. The MPK Mini is going to be used to play for example the piano instrument bundled in Acid or any synthesizers it may come with with.
So you'll need to look in the manual for one, how to make sure that Acid knows your MPK is hooked up over USB. That is likely in MIDI settings in some kind of preference settings from the program toolbar in the upper left of the screen.
Then you'll need to have a MIDI track created and load a VST sound generating instrument for that track to have something to play from the MPK. Acid isn't really designed to trigger samples from a MIDI keyboard or drum pads as far as I recall.
But there are plenty of free VST instruments that can load samples, as well as free synths and other things. Try KVR.com to see reviews and overviews of 1000's of free VST's. There are many Effects VSTs there too, so you can add to the reverbs and EQ's and delays that Acid includes.
http://www.kvraudio.com/And the specifics for an external MIDI controller controlling an instrument on a MIDI track are on page 39 of the Acid manual.
http://dspcdn.sonycreativesoftware.com/ ... sg_enu.pdf
