Using PayPal with Shoprocket is simple. Just login to your Shoprocket account and click on the settings link in the left hand menu.
From here, you can click the "Payment Gateways" tab:
Just add your PayPal email address into the PayPal field and ensure it is marked as "Active".
That's it! Now, your customers can pay with PayPal during checkout and the funds will be sent to your PayPal account immediately, minus the Shoprocket fee.