After changing the settings to Advance Mode and Javascript enabled, I can use the SignIn, but I get an error message, "A potentially dangerous Request.Cookies value was detected from the client . . . and processing of the request has been aborted." It takes about 5 minutes to get to that point.
When I called Verizon Support, the person had never heard of the Kindle. He thought it probably simply didn't have the power to handle the site, which is slow even on a regular PC. He said they are going to set up a special portal for mobile users which will be much less resource intensive, but he didn't know when it would be available.