If you've done those things, and you can't figure it out, go to our main website, and on the top heading where it says you can download latest version of Infiltration, then you can play online here or here- those two 'here's link to our dedicated SOB Infiltration servers online. A game should automatically load up!
Another way is to load up Unreal, and while connected to the internet, select the Multiplayer option, then Join- a window will appear where you can choose to look at all servers- just search for any telling you that they are playing Infiltration!