With a Checkout custom-coded solution, there is an additional step: set SOLUTIONTYPE to Sole in your initial SetExpressCheckout API call.
If you use a third-party shopping cart, work with them or your developer to implement this variable.
As long as SOLUTIONTYPE=Sole is passed in the SetExpressCheckout API, Guest Checkout will be enabled.
For SOLUTIONTYPE=Sole to work, you must have PayPal Account Optional set to On within the PayPal account settings.
Many third-party carts provide an Enable Guest Checkout option in the Administration view of their Checkout module.