I'm going to say it (somewhat) nicely before someone else comes around and doesn't... If you don't know how to restore from iTunes you shouldn't be doing anything involving development software.
First, get the UDID of your device and add that to the provisioning portal at the Apple dev site.
Then open up iTunes, alt-click on Restore, and then choose the beta ipsw you downloaded from the developer site. Warning: A lot of people (myself included) who have recently registered a new device under the provisioning protocol are getting false errors during activation. In other words, we can't get past the activation stage. The dev forums are down too. So you'll be "bricking" your device until Apple's UDID device registration servers catch up with the demand.