[Linuxephus™]

-snip-

*Image*
The above link shows that Ubuntu 12.04-LTS currently does not have Aircrack-ng in it's default Repositories.
You might want to edit your tutorial to reflect this by pointing users of Linux to the Aircrack-ng PPA for Launchpad or a suitable alternative PPA for it mate.
Just to avoid any confusion for those who might discover that the hard way.
Footnote: Staff is free to delete this comment after Mr.Grayhat has reflected this in his tutorial should he choose to.