If you have not yet downloaded ActiveInbox, please get it from the Install page.
You can install it on every computer you access Gmail from (your ActiveInbox Plus account is licensed per Gmail account, and you can use it on as many computers or browsers as you wish).
If it says 'Plus' on the right-hand side of the top Review Bar, you're all set up! (This is most likely because you were already registered during the Trial).
Otherwise, click 'Register...' on the right-hand side of the top Review Bar and then enter your ActiveInbox Acccount password.
Note that your ActiveInbox Account email address must match the email address in the top right of your Gmail account - i.e. the full email address you log into Gmail with. If it doesn't match, you can change your registered account email address using the Manage Account page.
(Please be aware that if you're in Europe you may have an email address that ends with '@googlemail.com' rather than '@gmail.com').