It was a bit more straightforward with the old SE Account Management pages... big button on the left for registering/updating payment method. Think you can still use that to manage your CC details... not sure, as I use it just for maintaining my Crysta Passbook.

In the Mog Station though, you should be able to get to it by clicking on the Account Services button. This should get you to a button for "Recurring Subscription Setup". There should be a button for redeeming a Game Time Card if you are going that route, otherwise you will need to click off the options in the table below.

At the top of the table, you will have to select your payment plan (Standard or Entry--determines how many characters per world you can have, you pay more for more toons per world). Below that you can select specific Credit Card Options or Crysta. If you select Crysta, this gives you the option of purchasing Crysta through Ultimate Game Cards (UGC), PayPal, etc. via the Ultimate Pay options. Haven't looked at adding Crysta via Mog Station, so can't give tips there. I still use the old SE AM page I linked above for adding my Crysta, simply because that's how I did it in XI. It shows my Crysta balance on login, and has a button to go directly to adding Crysta--pretty straight forward that way.