More generally, the following method should work for every repository. First of all search, with eventual help of a search engine, for a text on the program provider's website looking like the following:
-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: GnuPG v1.4.1 (GNU/Linux)
-----END PGP PUBLIC KEY BLOCK-----
Such a text is for example displayed on http://deb.opera.com. Copy the passage, paste it in an empty file that you create on your desktop. This results in the key file.
Then continue with the importation of the key:
- Applications > Sofware Center
- Edit > Sofware sources..., enter password
- Authentication tab, click on 'Import Key File...'
- Select the saved key file and click on 'Ok'.
You may now remove the previously created key file.