How to log in to BackTrack Linux?

I've installed BackTrack 5 R3, a security testing distro, onto a USB drive, and I can get it to boot up, but I'm stuck in the BusyBox shell. Everybody says "just log in with username root and password toor, then type startx to launch the GUI, but startx reports "command not found". I've tried various commands such as "login" and "sh", but no luck. sudo is out of the question, too. I suspect that I need to mount the system squashfs somehow, but I can't figure out how I'm supposed to do it. I haven't had much experience with SFS's before.