Hello ddog2000 -
We are not aware of any difficulties on the site that would cause such an issue with the popup to add a credit card to your account. I tested this functionality in my own account and was unable to replicate any issues with adding a new card.
I'd recommend that you first try clearing your browser cache and then return to the site to try again. If the difficulty persists, please file a support case from under the Help tab of the site letting us know what occurs for you when you try with a clear cache as well as your Operating System and Browser information so we can look into it further.