Two problems here.
First is a bug I'm pretty sure: the full title of a collection of Bibles that I have does not match in the dropdown list in Text Comparison (see screencast for a demonstration of this).
Second is a UI weirdness or design flaw. It seems you cannot get the item you type into the Resources box in the Text Comparison tool to "take" unless you click the blue arrow icon. Hitting Enter should be equivalent to clicking the blue arrow icon, but I try it repeatedly in the video below, and it doesn't do anything. At one point in the video, I see the focus highlight around the A icon for Show Differences, which would explain in that instance why Enter does nothing, but it does nothing even when the insertion point is clearly sitting there after whatever I'd typed in the Resources box.
Here's the screencast:
http://www.screencast.com/t/ylgFtIhBa