First thing to check, do you have the latest version of Circle installed? It's always available here: http://www.futureaudioworkshop.com/circle/downloads/
If that doesn't work...
Each activation code comes with three activation slots, allowing for installation on three computers. Once your activation slots are filled, when you try to activate Circle you will be shown "Serial in use".
But don't worry! Just drop us a line at firstname.lastname@example.org and we'll add more activation slots to your account right away.