In your Dashboard, click Buy Energy. Then enter the amount of EB's you would like to buy. You can purchase 1 EB for 0.60 Credits, which will automatically give you 150 Energy Points and a Unit in the Garden.
You can also set up Auto Buy. Auto Buy will purchase the maximum allowed EB's daily, which is 20. This will charge your account automatically the amount of credits needed.
The Auto Buy Feature is also under the RRS system. Therefore if your Credits balance falls to 200 Credits, the Auto Buy feature will not purchase any more EB's.