Greg Reynolds from the Canadian Air and Space Museum in Ottawa has provided a tutorial on how add GPIO based buttons to a Radiobuttonshow. He provides much useful information which is not immediately obvious from the manual including the directory structure of Pi Presents. He takes you through editing gpio.cfg and a copy of the pp_radiobuttonshow_1p3 example profile to show how to set up the control path between a button click and a video playing.
The tutorial is applicable to Version 1.3 (gapless). Version 1.2 (next) has detailed differences but the genral principles are the same.
The tutorial is a pdf document which can be found here