Once you have applied to and been approved through Stripe, go to E-commerce > Payment Options.
Scroll down to the Stripe box and click Multi Currency.
Add in the API Secret Key and the API Publishable Key given to you by Stripe. You can access this via your Stripe dashboard, or it is normally in the approval email. It is best to copy and paste these. Ensure there are no extra spaces before or after the key.
You can test by making a hidden product for say $1 and checking out as if you were a customer.