It's in the Options menu. Go to the Interface Options tab, and look for the option labeled "Use small scale interface". It should be checked ON by default; turning it off makes the game use UnitPanel.xml. Or, check the one right above it, "automatically size interface", and it'll decide which to use based on resolution.
My own mod has overhauled both of these, so I've spent far too much time going over the minutae of both of these.