If you haven't ever unpacked it, you don't have access to all the files available.
Open GCFscape and, in the "File" menu bar, select open. Navigate to STEAM/steamapps. Inside, you'll find your "email" folder and your mod gcf files. Select one of those gcf files to open. Once it's open, you'll see a traditional two-sided browser. From here, you can select a file and right-click it. Select "extract" from the menu and it will ask you to choose a location to extract it to.
It's just that easy.