I just created this SP level. And I want to add the Darth Vader 2.2 NPC to my map, only I'm not quiet familiar on how to do that. I know that you would normally in radiant you would right click and go to NPC and add which one you want. But Vader is not on the list, so how do you add him or someone else to the map.
Use the NPC_Spawner entity to spawn npc's not on the entity list. If you just want it to spawn one from the start, set the NPC_type key. The value for this key is the name you'd use to spawn the npc from the console.
It functions similarly to regular NPC_XXXX entities, if you target it. Left on it's own it will just spawn one guy, if triggered it can be told to spawn at a specific time. Even better though, you can make an NPC_Spawner spawn repeatedly. You can create infinite loops, making it so when one dies, it retriggers the spawner, and a new NPC is dropped out. You can make it respawn up to a certain number similarly.
You should read the definition for the entity in the entity window, there are many advanced features that the NPC_Spawner can do, that may have been a boat-load more complicated using the regular NPC function.
Learn it, love it, enjoy!