Welcome to the forums, Defcon5
You can't actually attach a flag to a vehicle... the trick is to give it to someone who's
inside the vehicle
If you create a flagpole, and add something like this to it's init line:
this setflagtexture "
texture";
unit setflagowner this
Where
texture is the texture of your flag (eg. "usa_vlajka.pac" for US, "rus_vlajka.pac" for russian... there are many more, just ask
).
Unit is the unit you want to have the flag. If you simply have a vehicle, you can use: (driver
vehicle).
If your vehicle is empty, we may have do come up with something a bit more cunning...