Go to your tools tab in Steam, if you see 'Source SDK Base 2007' then install it. You need this installed to play GE:S.
However, if free Portal is the only source game you have then you will not have Source SDK Base, which is required to both play and develop Source mods. It isn't available with free editions of Source.
If this is the case you'll need to purchase a Valve Source game on your Steam account. If you're looking to pay very little, then Half-Life 2 Deathmatch is incredibly cheap. If you want the most bang for your buck then buy the Orange Box, 5 great source games (including portal) for a great price.
Also, if you're using a Mac, forget it, there's no Mac support on any mods yet.