I'm on my iPhone right now so I can't actually get te exact link for you, but I'm fairly certain that if you go to the iPhone developer portal there's a walkthrough that gives the exact steps to begin testing on a device...
I'll edit with an exact link when I get home in a half hour if no one else gives it by then and you can't find it...
Edit: Log into the iOS developer center, on the top right of the page, click on "iOS Provisioning Portal" and this should take you to the home section of the Provision Portal. Front and center on this page it should have a button that says "Launch Assistant". Click on that, and it'll walk you through the process of being able to run your iOS apps on your iOS devices to test them.