Are you trying to add an AP
gauge?
You can try manually editing the panel.cfg file but it's easier to get CfgEdit (
http://www.cfgedit.com/).
Then make sure your aircraft.cfg file is set up to handle it, for example (taken from 777):
(autopilot) <--Brackets instead of parens
autopilot_available=1
flight_director_available=1
default_vertical_speed=1800.0
autothrottle_available=1
autothrottle_arming_required=1
autothrottle_max_rpm=90
autothrottle_takeoff_ga=1
pitch_takeoff_ga=8;