you need a Source engine game such as Counter-Strike: Source, Half-Life 2, or Day of Defeat: Source.
Please note that Half-Life 2: Deathmatch does not fulfill this requirement.
yeah well not everybody can download it from steam, especially with problems like "steam servers too busy to handle ur request" and stuff like that. real bugger. just taking the load off steam
yeah and when i say buy i mean half life 2 by the way ; just so u know, i'm not exactly stupid geez . . .