Download the Nessus package from http://www.tenable.com/products/nessus/ and install it manually. Read my post before about How to Install Package on Backtrack 5 after you install it, skip to step two. If you have internet connection you can install Nessus from repository. Run this command to download and install Nessus.
apt-get install nessus
After install it, create an account with adduser command like this
/opt/nessus/sbin/nessus-adduser
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
It will take some time because plugin is being updated. After the update complete, run your Nessus
/etc/init.d/nessusd start
Then open your browser and type this in the URL of the browser
https://localhost:8834/
Nessus will run on the secure channel https and on the port number 8834. Now try and use Nessus for your own risk :-D