Center "N" Target is switch 28.
Center "N" lamp in MANIA is driven by Q8 on the lamp driver board.
http://www.pinwiki.com/wiki/index.php?title=Silverball_Mania
Try putting in a know to be good regular light bulb first before putting in a led.
The power is supplied via the bare wire at the mounting screw of the lamp socket.
If you do not have any voltage at the mounting screw, check the bare wire.
Put in a regular light bulb and measure at the small wire at the tip of the socket.
If the lamp is good, you should have 6.5V at this point.
Please report back your findings...