C:\Program Files\Valve\Steam\SteamApps\SourceMods

this is where all mine are located you just need
to make sure the mods folder is in there with
all its programs and stuff while steam
is not started then start up steam offline
and it should be there on your games list.
I normally use a bare bones start up with
Windows xp nothing but Microsoft processes running in the
background.
So Steam is not running anything when I place
the mods into that folder above. Not sure that makes any difference.

buddyboy