In an earlier post I described how to setup Nessus on Backtrack 5:
For some reason, BT 5 R1 does not come with Nessus installed by default, but it's easy to download and install.
You will need to download Nessus from the following location:
(I used the Debian 64 bit package)
Then install it with the following command:
dpkg -i Nessus-4.4.1-debian5_amd64.deb
Then you can simply follow the rest of the instructions in my original post.
Update - Of course, there is an easier way to do this ;o)
apt-get install nessus