you need a good multi track recording program. There are alot of them out there -
if you want to pay Nuendo is amazing....Adobe Audition is really nice but for use with the rp250 (i have this pedal and have gotten some really good results with it) i recommend Audacity.
It supports the Aiso interface that the rp requires - It's free - and supprisingly powerful and easy to use. Download and see if you can figure it out if not I will give you some step by step instructions.
http://audacity.sourceforge.net/