[Update 2:40 PM PDT] The issue has been resolved, "Add Balance" now functions for all payment methods. Thank you for your patience.
We are currently experiencing an issue with some of our back-end billing systems, and as a result the "Add Balance" function for all payment methods besides Nexon Cards and Epin are currently disabled. We are looking into the issue, and will update this post with further information as it becomes available. Thank you for your patience.
- The MapleStory Team