Error: unable to open display ":0.0"

[aerotao]

I've had a problem with running flightgear 2.4.0 on Ubuntu 11.04.
When I tried running it shows
Error: unable to open display ":0.0"
Error: unable to open display ":0.0"

Then the screens setting is not show, means that I set up the multiscreen on the flightgear preference to 3 screens but it shows only one screen.
---I'm using Nvidia GTX 560---