The synaptics version of Nessus its on version 2 (opensource), on official website of Nessus you can get the latest version.
1) download Nessus and NessusClient depending of your distro and your processor arch
2) reproduce the following steps
# sudo dpkg -i Nessus-XXXXX.deb
# sudo dpkg -i NessusClient-XXXXX.deb
3) Get an activation code for update nessus plugins: register in nessus website at plugins section, click on request an activation code, then click on Register a HomeFeed, then go to your email
# /opt/nessus/bin/nessus-fetch --register XXXX-XXXX-XXXX-XXXX-XXXX
4) adding an user, on Authentication (pass/cert) you can press enter ...
5) starting nessus deamon (server)
# sudo /etc/init.d/nessusd start
6) now its time to run nessus client (gui), also you can run from NessusClient shortcut (look for your desktop environment applications menu)