This error occurs when you try to compile a model/texture and Steam does not know what game you are compiling for. This happens sometimes after a Steam update. It also happens for certain games such as Left 4 Dead.

The first solution (if this is following a Steam update) is to restart Steam and run Source SDK... then trying to compile your model again.

The other solution is to explicitly state what game you are compiling for. To do this in WWMT, go to the settings floater and set the Game Info Dir that was added in version 0.999.