The Credits window can be accessed using the dropdown arrow in the Org section (top status bar).
The window includes detailed information about your usage and availability of credits
In the upper right-hand corner of the window, you will see an ADD CREDITS button. Press this button to refill credits.
Note: Credits expire 12 months from the date purchased. Credits may only be purchased with a credit card at this time.
Refer to the pricing page for the most recent information about credits cost.