Ubuntu Natty
You can update your system by adding ppa:backbox/two to your system's software sources.
Step 1 - Open a terminal and enter:
sudo add-apt-repository ppa:backbox/two
Your system will now fetch the PPA's key. This enables your Ubuntu system to verify that the packages in the PPA have not been interfered with since they were built.
Step 2 - Now, as a one-off, you should tell your system to pull down the latest list of software from each archive it knows about, including the PPA you just added:
sudo apt-get update
Now you're ready to start installing software!
Other systems
This PPA also can be added to your system manually by copying the lines below and adding them to your system's software sources.
Step 1 - Open a terminal and type:
sudo nano /etc/apt/sources.list
This will open a text editor containing the list of archives that your system is currently using. Scroll to the bottom of the file and paste the lines you copied in the step above. Save the file and exit the text editor.
Step 2 - Now you need to add that key to your system so Ubuntu can verify the packages from the PPA. In your terminal, enter:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 78A7ABE1
This will now pull down the PPA's key and add it to your system.
Step 3 - Now, as a one-off, you should tell your system to pull down the latest list of software from each archive it knows about, including the PPA you just added:
sudo apt-get update
Now you're ready to start installing software!