When your credits run out you get a notification in the Filestar desktop application, the next time you try to run a skill or a file conversion. Just follow the simple steps there to load up on more credits.
It is also easy to buy more credits before they run out, or upgrade your package. Go to our pricing page, use the e-mail address that you have registered and select how many credits you want to buy.
You can always check your credit account balance in the desktop application if you click on "About" in the menu.