To activate/connect a Stripe account to your PTminder account, head over to the Settings area then to the Payment Integration tab.
Once you've read and accepted the charges, select the 'Connect with Stripe' button:
It will take you through a few steps to authorise your Stripe account with PTminder. If you have an existing Stripe account, you can login with that, otherwise you will be able to sign up for a new account on the spot.
What is Stripe?