"This Steam account does not own this game."
 

I'll leave this stickied for a while as we've been seeing a large influx of questions regarding this problem.

If you get an error about not owning the game when trying to run SourceForts, it becuase you do not have a valid copy of Half-Life 2: Deathmatch. This game is required to run and Half-Life 2 on it's own will not suffice. You will also receive this error if your HL2: DM comes from the ATI or NVIDIA promotion, as you are unable to download the Source SDK Base with that version.