I figured it out by reading the 'Getting to know Directory Opus' tutorial.
What? How novel... That's something that I almost always neglect to do (seriously! I'm always amazed when someone says "I solved the problem" and then explain that they read the manual, or worse, the "Quick Installation Guide", etc. because I realise too late that this is something that I haven't even thought of doing!).
Good for you and thanks for posting the solution. Interesting that the discussion is about the buttons - I have removed almost all of them from my toolbar and use the context menu. Thus when I checked into this earlier, I was using the context menu, not the buttons (which are not present on my toolbar).
Any idea what's the equivalent of XY's Catalog in DO? i.e., I want to add (a pointer to) a file or folder and I should be able to click to go to that location in DO (followed by a press of return key to open).
Haven't used this feature of XY, but suspect that what you are after in DOpus are "File Collections" (something else I really haven't used).
As for adding individual files to the Favorites, I don't really know - I've never thought to ask that question!