Ok, I'm stumped on this one. How on earth do you actually stream content on http://stream.nbcsports.com/liveextra/

I can't see a login button, or any way of actually watching content?!

click on replays on the list of events at the bottom of the page, also set your pc time to eastern time (usa) or the live events wont be shown as available

Click on the event which will take you into some kind of player with nbc sports logo in the middle. Click on that logo and it will ask you to choose a provider which will take you to the login screen.
In some cases there will be no logo and you can view the streaming without resorting to logging in via the provider. In most cases the streaming will not work (geo blocked) unless you are coming from an US ip address and live events are not available to access until their time comes.
Your best bet is to use the replay functionality.. If you have an ipad, i recommend downloading their app.