Releasing A Portal 2 Map
Now that you have made your brilliant map you need a way to get this out to the public. If your map happens to have custom content such as sounds, scripts and materials the best way to release it is putting it inside a .VPK. This zips all your content up into a single file which makes it easy to distribute.
Creating the VPK
1. Create a folder and label it with your addon name.
2. This folder should now mimic the structure of the 'Portal 2\portal2\' folder.
3. Start with adding a folder called 'maps' and add in your finished .BSP.
4. If you have any scripts be sure it lies in the same relative folder inside "scripts\vscripts".
5. The same applies for sounds and materials.
6. Inside 'Portal 2\bin\' there is an executable called 'vpk.exe'. Drag your folder you created in step 1 onto this executable.
7. You will now have a .vpk file in the same folder with the same name.
Installing the VPK
1. Create a folder in 'Portal 2\portal2\' called 'addons'.
2. Place your .vpk file inside the addons folder.
3a. If Portal 2 was not running during step 2, load portal 2 and head to step 4.
3b. If Portal 2 was running during step 2, type the following in the console: 'update_addon_paths'.
4. To load a single player map, type in the console: 'map mymap' replace mymap with the name of your map.
5. To load a Co-op map, start a Co-op game with your friend, once you enter the hub type in console: 'changelevel mymap' replace mymap with the name of your map.
Left 4 dead 2 uses the same technique for it's addons.
http://www.youtube.com/watch?v=tDAF_ZOQx1U - How to make custom VPK.