No overlay on osu!
Is there a way to force the overlay onto games where it won't appear?
Or do I have to select osu! as a game?
FBX will automatically look for specific processes to identify if what you have on screen is a game or not. If it detects that it is, the overlay should appear on the game when it is either the active window or running fullscreen - so just be aware that if you are playing in windowed mode and click on to another window, the overlay will temporarily disappear until you click back on to the game window.
We've tried out OSU! and FBX detected and recorded it no problem, so if you are not seeing the overlay when you are in the game, can you use the 'Send a problem report' on FBX's Support tab to send us a a set of log files please? We'll take a look at those to try to figure out what's going on.