what version of backtrack are you on?
when you first run autopsy setup from the b|t menu, you should have been prompted to allocate a directory for your "evidence locker". make a directory in another shell and then give autopsy it's location there on the system.
Autopsy Forensic Browser
(example) Evidence Locker: /root/evidence
Start Time: Mon Aug 15 22:36:46 2011
Remote Host: localhost
Local Port: 9999
Open an HTML browser on the remote host and paste this URL in it:
Keep this process running and use <ctrl-c> to exit
what did you do differently?