Serial communication

Hi i'm trying to make arduino work for flightgear, but it seems the arduino COM is busy, i tried to plug and unplug arduino but it didn't work, i've also noticed when i plug arduino on my pc with no programs opened the tx led on arduino is working, but not on other computer. (sorry for my bad english)

Here is some reference material that should help: http://lmgtfy.com/?q=arduino+flightgear