As long as you have..
An DX10 Compatible graphics card (Nvidia 7950 and 8 & 9 Series and a few ATI's)
SP1 & SP2 Installed
You will see a tickbox in the graphics settings on FSX to turn on DX10. You will need to restart FSX to enable this.
You might see flashing taxiways, particularly on intersections. This is a clash of some sort and MS
have ignored it.