Add Missing apt-get Public Key

After updating your list of software sources within update manager, you may receive the following message:

'The action would require the installation of packages from non-authenticated sources'

To resolve, perform the following:

1. Open a terminal and run:
sudo apt-get update
2. Then take note of all the numbers (keys) after ‘NO_PUBKEY’
W: GPG error: http://ppa.launchpad.net oneiric Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY XXXXXXXXXXXXXXXX
3. Run the following for each missing public key:
sudo apt-key adv --recv-key --keyserver keyserver.ubuntu.com XXXXXXXXXXXXXXXX
4. Finally, run the following:
sudo apt-get update

After this, you should be able to install the update through update manager without any problems