How do you make NPC's show up in the spawnlist?

OK, I consider this separate from my other thread, so I will go ahead and ask the question:

How do you make NPC’s show up in the spawnlist? Do I have to add them to a file or something? Because whenever I go to the menu to spawn a NPC they don’t show up!

I am only talking about downloaded NPC’s.

Again, you either installed them wrong or they got broken.

Really? It is the same thing?