your username is unknown to sudo

Type:

Code:

su

then

Code:

visudo

and add yourself to the sudoers.

You'll see a line like that:

Code:

root    ALL=(ALL) ALL

add one just below with your username...

Code:

username    ALL=(ALL) ALL

and that's the end of it.

Source: http://forum.ubuntu-fr.org/viewtopic.php?id=75743