This could be because of anti-virus/security software you have installed on your computer. Some security programs might interpret the launch of NFL SUNDAY TICKET Online as a pop-up. If you disable pop-ups and ads through your anti-virus/security software (not just in your browser), you should get NFL SUNDAY TICKET Online service to work.
Once you have NFL SUNDAY TICKET Online launched and it's working, you should be able to turn security back on. You also might consider making NFL SUNDAY TICKET Online an exception site that doesn't get the anti-virus/security rules applied to it.
For more information about NFL SUNDAY TICKET, visit directv.com/NFL