You must have this mod to play (also include a portal gun model)
http://www.mediafire.com/?3uc96jdifsr8skq

RULES:
-You can only create a portal on a white wall.
-You can't create a portal on any other wall (different color, gates, fences,...)
-You can't create a portal on the ceiling. (the roof)
-Please follow the rules!
-And the most important thing is...DO NOT CHEAT!

If you get stuck somewhere, go here and ask or checkout the walkthrough if available.
http://www.gtaforums.com/index.php?showtopic=512761