From Aeria Browser Wiki
Buying DDTank Coins:
To purchase coins in DDTank, first you will need to purchase Aeria Points. Purchasing Aeria Points will allow you to get the most out of your Aeria Games experience. The Aeria Points that you purchase will show up in the in-game Item Mall and allow you to purchase cool and useful items for any of our games. So start experiencing more from our games, and enjoy! Please read the billing FAQ before buying.
For a detailed explanation on how to purchase coins, please check out this thread! http://www.aeriagames.com/forums/en/viewtopic.php?t=1163151