Open up Source SDK, select Half Life 2: Episode 1 as the engine version and go into Hammer. Select "Tools > Options..." In the Game Configs tab and press edit near the drop down menu for Configuration. Now press add and type in Platform Source. Now press on the drop down menu and select Platform Source. If you don't then you might be editing configuration for other games and, you dont want to do that. Now you need to add Game Data File, press on add, go to Steam, Steam Apps, Your Username, Sourcesdk, bin, ep1, bin and select halflife2.fgd. Now make sure that, the texture format is set to Materials (HL2), The map type is set to HL2, the Defalt Point Entity class is set to ai_ally_manager, Defalt Solid Entity Class is set to func_detail, the defalt texture scale doesn't matter but I just use the defalt which is 0.25, the Defalt Lightmap Scale is 16, The cordon Texture is set to tools\toolsskybox, The Game Exe file is set to $SteamUserDir\half-life 2, The game dir is set to $SteamDir\steamapps\SourceMods\Platform Source, and the hammer VMF dir is set to $SteamDir\steamapps\SourceMods\sourcesdk_content\Platform Source\mapsrc.
Now you need to press on the last tab which is Build Programs. Press on the Config dropdown menu and select Platform Source, the game exe should be set to $SteamUserDir\half-life 2\hl2.exe, The BSP exe should be set to $SteamUserDir\sourcesdk\bin\ep1\bin\vbsp.exe, The VIS exe should be set to $SteamUserDir\sourcesdk\bin\ep1\bin\vvis.exe, the RAD exe should be set to $SteamUserDir\sourcesdk\bin\ep1\bin\vrad.exe and finally, the lsat thing is the place where it should put the maps when you compile them, set this to $SteamDir\steamapps\SourceMods\Platform Source\maps. When you press ok it will load a bit, you will have to exit it and now in Source SDK you will be able to choose Platform Source.
Note: None of this will work if you dont have Platform Source in your SourceMods directory and therefor most people wont be able to use this tut for now.
