You do not need to own a PayPal account to pay: you may enter your credit card's data directly at PayPal and complete your payment. You don't need to register a PayPal account neither.
Once you get redirected to PayPal, just do as follows:
If your PayPal page looks as follows, just click on "Pay with my debit or credit card".
Address, email & phone number may also be requested depending on your country.
When you are done, click on "Pay".
If you run into any trouble or have any questions about PayPal's Guest Checkout, please contact PayPal directly.
PayPal's phone numbers are: 1-888-221-1161 (US), 1-402-935-2050 (if calling from outside the US)