When trying to add folders to the source, the "OK" button is greyed-out.

Hi,
Please make sure you pressed "Add Folder" button, not the "Add files" button.

This is not a suitable answer. I'll repeat the question: "When trying to add folders to the source, the "OK" button is greyed-out." Under what conditions would this occur? Make the assumption that the "Add Folder" button was pressed, and a folder in the hierarchy was selected. This was a serious question.

Hi,
There are no exceptions for the OK button, it should work all the time.
Do you have the same problem for any other folder you are trying to add?

If not, can you tell me the exact path of the folder you are trying to add?

It's probably because that folder has already been added in another source entry. I just had the same problem and it was because it was a duplicate entry.