If you have missed the original activation link sent by your school for your Calculus account, you can request a new link to be sent to your email address.
Simply click on the expired activation link sent to you via email by your school. You will be prompted to this screen:
Click on 'Request New Reset Link' and then set up your password:
Afterwards, wait a bit and check your email inbox for this kind of email:
In case you are unable to locate such email at first, please wait a bit more and also check your Spam/Junk folder.