AFAIK, it's not possible to 'sample' with Recycle...
1.) Trim the loop you want in your DAW.
2.) Bounce that as a Wav.
3.) Open Recycle, import Wav.
4.) Adjust sensitivity until you get the amount of slices you want your .rex file to have.
5.) Export Rex file.
6.) If you want to open that Rex in N.I. Battery (my first choice of Rex playback device), you should also export a midi file from Recycle... you can then open the Rex in Battery as though it were a kit, then drag the midi to your DAw to trigger it.