12.04 - Software Center gives me an "Error scanning the CD: Could not find a suitable CD"

I am getting the "Could not find a suitable CD" error every time I open Software Sources in order to add a PPA. A picture can be found below:

How do I solve this problem? I am using Ubuntu 12.04

I see your mouse is near Add volume. If you are clicking it, that is the wrong button. You need to click Add near the left instead.