I did exactly like you said but it didn't tell me anything.
[drew@localhost drew]$ su
[root@localhost drew]# updatedb && locate gtk-config
So I tried to install that program again and this is how that went...
[drew@localhost security]$ sh nessus-installer.sh
The command 'gtk-config' was not found in your $PATH.
The nessus client will be built without its GUI...
Press ENTER to continue...