How to add the source of Ubuntu Tweak (The easy way, work since Ubuntu 9.10):
Go to System / Administration / Software Sources, click on Other Software tab, click Add button, and enter “ppa:tualatrix/ppa”.
This will add the PPA repo and the GPG key automatically.
You can also do it from command line by running:
sudo add-apt-repository ppa:tualatrix/ppa