For the home version of Nessus, yes you do have to register. Go back to tenable.com, register, and they email you the code to use. Then, on the BT5 machine the command is
backtrack#: /opt/nessus/bin/./nessus-fetch --register <code from email>
That will install the plugins........
However, BT5 already comes with the OpenVAS scanner, the open source fork of nessus.