Find your GameConfig.txt file.
It's located in ...\STEAM\SteamApps\common\SourceSDK\bin\source2007\bin
Open it up with a text editor.
If it's empty it should look like this:
"Configs"
{
"Games"
{
}
"SDKVersion" "3"
}
Inbetween the brackets, insert this Text:
"GE:S v5.0"
{
"GameDir" "D:\STEAM\SteamApps\sourcemods\gesource"
"hammer"
{
"GameData0" "D:\STEAM\SteamApps\sourcemods\gesource\goldeneye.fgd"
"TextureFormat" "5"
"MapFormat" "4"
"DefaultTextureScale" "0.250000"
"DefaultLightmapScale" "16"
"GameExe" "D:\STEAM\steamapps\common\Source SDK Base 2007\hl2.exe"
"DefaultSolidEntity" "func_detail"
"DefaultPointEntity" "info_player_deathmatch"
"BSP" "D:\STEAM\steamapps\common\SourceSDK\bin\source2007\bin\vbsp.exe"
"Vis" "D:\STEAM\steamapps\common\sourcesdk\bin\source2007\bin\vvis.exe"
"Light" "D:\STEAM\steamapps\common\sourcesdk\bin\source2007\bin\vrad.exe"
"GameExeDir" "D:\STEAM\SteamApps\sourcemods\gesource"
"MapDir" "D:\STEAM\SteamApps\sourcemods\gesource\mapsrc"
"BSPDir" "D:\STEAM\SteamApps\sourcemods\gesource\maps"
"CordonTexture" "tools\toolsskybox"
"MaterialExcludeCount" "0"
}
}
Now you only need to change the file paths to whereever your steam is located, if it's not acidentally D:\ for you.
I'd recomend the search and replace function to minimize the risk of typos.
For MapDir you're free to choose any path you like to store your .vmf files in.