I had this problem that I wanted to enable the in-game purchases and had to buy some cartel coins on the website before doing that, then I bought 450 coins and still it didn't gave me the option to enable it.
I don't know if it has been posted above I have no time to read it all, sorry, anyway this is why I couldn't do it and how to solve this: when I payed on the website the first time with the credit card, I forgot to check the "save data for next time whatever check box", so it will ask you again to buy something more when you want to enable the in-game purchases.
SOLUTION Now, simply go to your account details and add the same credit card on file. The next time you click "enable" it will show you the option to choose that card and it will enable the ingame purchase.
I hope this will help those that had the same issue.
Regards,
K.