From my experience, the issue comes from the user id. Most people that have this problem had a VZ username prior to setting up their FiOS TV service and they are trying to use that username, without having done the activation via the website with their order number.
So if that sounds like it might be you, then go to http://activate.verizon.net and use your order number and zip code to complete the activation, and then you should be able to use your FiOS TV Online services.
Please try it and report back if it's still giving you an issue.
If all of that is good, make sure you can access your bill online, and then try to access btn2go at this website, www.verizon.com/fiostvcentral and let us know what it does.
and fwiw, I can access it through the link you shared, I had to accept a browser plug in, and I just used my verizon log in to access it. also let us know if you get specific errors.