I downloaded the Ubuntu 11.10 / Ubuntu 12.04 (64 bits): Nessus-5.0.1-ubuntu1110_amd64.deb (25298 KB) from the following page:-
opened up the file using Ubuntu Software Centre and clicked on the install button on it's right. It prompted for a password. I entered the password and the installation began. After the installation was completed, the install button appeared again. Tried to search the Nessus in Dashboard Home, but no success. Help on this please !