How To Install Nessus On Backtrack 5

The first you must register for activation code, please visit http://www.tenable.com/products/nessus/nessus-homefeed
Then you can install via online :
Open terminal and type apt-get install nessus

Then you must add user for your nessus
type the command : /opt/nessus/sbin/nessus-adduser

next step enter the activation code from your email 
type the command : /opt/nessus/bin/nessus-fetch --register (your activation code)

You need to start nessus, type command : /etc/init.d/nessusd start

Then you must test on browsers use link localhost:8834

Enter the login and password

Done, Good Luck ^^