How do i take a power tab file (.ptb) and convert it to a .zip file?
Epi LP--->Digitech RP3--->Crate GT-200
you don't convert it - you compress it to zip!

Well, if you have Win XP it's easy :
Right click on the file -> Send to -> Compressed Zip folder

That's it!

If you have another OS then install WinZip or WinRAR and do it with those programs