As a lot of people have pointed out and I noticed is that Nessus is not installed in Backtrack anymore by default. This is such a powerful tool so you are going to want to add it back to Backtrack. This is a Step-By-Step guide on how to install Nessus.
1. Install Nessus
apt-get install nessus
2. After install it, create an account with adduser command like this
/opt/nessus/sbin/nessus-adduser
3. Now register to Nessus website http://www.nessus.org/register/ to get your activation code that send to your email. After you get the key, run this command, fill change xxxx-xxxx-xxxx-xxxx-xxxx with your key
/opt/nessus/bin/nessus-fetch –register xxxx-xxxx-xxxx-xxxx-xxxx
4. It will take some time because plugin is being updated. After the update complete, run your Nessus
/etc/init.d/nessusd start
5. Then open your browser and type this in the URL of the browser. Nessus will run on the secure channel https and on the port number 8834.