I changed my plan a few hours ago and I'm now seeing some funny things on "My Account" (it shows I have 6.00 GB available for use but the plan details have not changed). It also tries to refresh the details and then an error would occur. It did say that it would take up to 48 hours to see the changes reflected on the account so I'm hoping by then it would clear up. I also didn't get a confirmation email either.
In the Change My Plan option, try clicking the back arrow button instead of forward. I was only seeing the normal plans as well, but when I clicked the back arrow a few times, the BYOP plans finally showed up. It should be the $60 5GB plan. According to their reps, the 5GB will be added automatically but will take a few days to show up.
I would keep clicking back all the way until you see a $60/5GB option. You can try clicking forward too, but that didn't work for me. Others are saying they still don't see the option at all, so it may vary by account! Good luck.