libusb is present in /sw, so I guess something I've installed with fink has put it there. rbutilqt's not seeing it there, however. I'll try installing it in a more accessible location.
I've noticed the mountpoint issues from the point of view of a user; is this getting /Volumes/ to show in the file browser? (It works fine if you type the path manually)
EDIT: That part works now. Thanks very much for the advice, and the fix. However, now it fails later, with the following messages:
/usr/bin/ld: for architecture ppc
/usr/bin/ld: can't locate file for: -lusb
collect2: ld returned 1 exit status
/usr/bin/ld: for architecture i386
/usr/bin/ld: can't locate file for: -lusb
collect2: ld returned 1 exit status
lipo: can't open input file: /var/tmp//ccRCklBj.out (No such file or directory)
Again, is this just me?
rbutilqt kinda is in need of a Mac developer 
I wish I had the coding skills to contribute. The best I can offer at the moment is testing.