Download the Nessus installer (.deb) file here
Open a terminal;
dpkg -i "Nessus.....deb"
When it's done installing, go HERE to get a Registration Code.
register nessus:
$ cd /opt/nessus/bin
$ ./nessus-fetch --register "xxxxxx-xxxxx-xxxxx-xxxxx"
(might take some time for updating plugins)
"xxxxxx-xxxxx-xxxxx-xxxxx" is the serial code that tenable sent you after registration.
After it finished the update:
$ service nessusd start
This will start your nessus daemon on Port 8834
https://127.0.0.1:8834
(It may ask to add security exception, do it if that's the case)
Now Add a New Nessus User. The first time Nessus will take a long time to 'initialize', just be patient.
Then you will see a login screen, Enter your login name and password there.
Have fun.