You do not use global sources to add a source. Those are a different thing, they are used to create a source that is 'shared' between scenes. It's something I regret naming like that, the naming is just too confusing, so this is entirely my fault. In fact, there are a lot of regrets I have about the user interface, even though I personally love the design for myself, it just doesn't work for most people.
To add a source, you right click the 'sources' box (bottom center white box) in the main window. My sincerest apologies about the confusion.
In newer versions we'll be changing how this work, and removing 'global sources'.