Evin though you said you bought a pre soldered and flashed Ardupilot Mega, it might be best as Chris suggested to try to re upload the latest firmware as this might fix the problem.
Have you tried to connect using the Mission planner after you have uploaded the latest firmware? I would recommend trying to connect first with the Mission Planner as it is specific for Ardupilot Mega, and is constantly being updated.
What PC and OS are you running on?
Let us know how you get on