Nessus is now no longer using the Nessus Client as in previous versions.
It uses a Web Interface to set up, scan and view reports.
To download the Nessus package:
Tenable Network Security
Choose Linux. > Accept the agreement. > Choose the version that is for your distribution.
You’ll want to choose “Nessus-4.2.2-ubuntu810_i386” if your using Backtrack