If you're talking about the North American version, 420, you have to go to ijji.com, register an account, and to play, you must play using your internet explorer browser. If you would prefer Firefox, just ask, and I'll provide a link in how to make it possible when it's not supposed to be.
Unfortunately, for some odd reason, the NA version just laggs alot for me, so I do not play it often, I stick around International from time to time, and I still get lagg, but it's server lagg and everyone else gets it too. I-GunZ just runs way better for me, for some reason. The whole browser/play deal, is fucking retarded in my opinion, and I do not like it.