Open a terminal window and type in: 
sudo apt-get install nessus 
sudo apt-get install nessusd 
sudo nessus-adduser 
sudo ln -fs /etc/init.d/nessusd /etc/rc2.d/S20nessusd 
sudo /etc/init.d/nessusd start 
sudo gedit /usr/share/applications/Nessus.desktop 
 
Insert the following lines into the new file 
 
[Desktop Entry] 
Name=Nessus 
Comment=Nessus 
Exec=nessus 
Icon=/usr/share/pixmaps/nessus.xpm 
Terminal=false 
Type=Application 
Categories=Application;System; 
 
After that you can find Nessus in the Gnome menu under Applications -> System Tools.