Garry's Mod 9 can't load single player

I have Garry’s Mod 9 (the Half-Life 2 mod) installed. I can launch it from the Source SDK, but when I try to launch it from Steam and click the Single Player or Create Server buttons, no dialog appears. It doesn’t crash. Originally it didn’t load sucessfully at all from Steam, so I applied this fix.
Does anyone know how to fix this?