A munkpower is achieved by getting four perfect moves (blue ticks). This will fill thed bar at the bottom right of the screen and then you will see in the move list one box will change to 'Munk Power', your chipmunk will do it's own move and then you are free to earn a power again.
I got this on Life is a Highway (i think). It is about half way through the story mode and has a relatively simple routine. I got 5 without too much effort.
EDIT: Thanks to takuto neko and CaMmY95 for confirming it. A song that they seem to be finding easier is Video killed the radio star so this is also worth trying out.