ok...heres what you need to do
1) make sure port
27015 is open on your router
2) before you open the game, press
~ on your keyboard. this will bring up the console
3) type
sv_lan 0 in the console. this will set lan mode to 0 (meaning internet game)
4) your external IP should be something like 71.36.106.49 (NOT EXACTLY, and NOT "192.168.x.xxx" thats internal) not "sourceforts.dyndns.org" (to find your ip go to
http://www.dnsstuff.com and scroll down to were it says "you seem to be in the -Country- based on you IP of xxx.xxx.xxx.xxx)
if you did all three right, then it should run
if not, you screwed something up