Is anything red? You need to power the transmitter and a console, and make sure that the timer is not off if you are using the existing generators inside.
You also have to set up a tower to call in settlers.
Under the http://fallout.wikia.com/wiki/The_Castle#Bugs section for The Castle
There's also a bug with the settler-summons bell I've heard about. If you build one and ring it, the radio man will come to the bell but there's a good chance he'll never return to his duties.