Go into the configs file (paintball2\pball\configs\config.cfg) and search for "gl_mode". If it isn't there (I don't think it is) you have to add "set gl_mode 4", if it is there just make it say '4' instead of what ever number is there.
You could also do this in-game by bringing up the console (hit "~") and type "gl_mode 4".