In the windows version, you can add previously made collections to a new collection by dragging and dropping the old collection into the "+ Plus these resources" box. In the mac version, that shortcut does not exist. Instead, you have to right-click and then click "add". Just a thought on a new feature in order to make them identical.
I'm filing a case for this. Thanks.
The ability to drag and drop collections from the "Open" collections drop-down list to the "+Plus these resources" and "-Minus these resources" boxes will be available in version 4.5, which may be available for beta testing in the next couple of weeks.