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
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
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.