Unable to add PPA repository from terminal

Since you can't add the repository, you can always add them from a terminal using the command line. Browse to the list of the repositories at the WebUpd8 Website. Copy down the address of the master repository, which is Master Repository. You want to add this one because it contains all the others.

1. sudo cp /etc/apt/sources.list /etc/apt/sources.list.backup
2. sudo gedit /etc/apt/sources.list
3. Visit Master Repository in a Web Browser.
4. Find the Dropdown Arrow that reads:

   Technical Details about this PPA

5. Click Your Ubuntu Version in the List Labeled Choose Your Version
6. Add the resulting output into the file in Step 2.
7. Save the File
8. sudo apt-get update. This update command should now fetch the Private Keys of the new Repository. If you receive the error as you stated in your comment, then this repository has no private key. You may want to contact the PPA maintainer at that point who will either give you the key, or tell you to ignore the Warning.