I've struggled with this question and found the answer following Roberts comment about using LANDINGPAGE=Billing in the ExpressCheckout API.
From the page:
https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_nvp_r_SetExpressCheckout
LANDINGPAGE:Type of PayPal page to display.
It is one of the following values:
- Billing – Non-PayPal account
- Login – PayPal account login
Additionally see:
SOLUTIONTYPE: Type of checkout flow. It is one of the following values:
- Sole – Buyer does not need to create a PayPal account to check out. This is referred to as PayPal Account Optional.
- Mark – Buyer must have a PayPal account to check out.
NOTE:You can pass Mark to selectively override the PayPal Account Optional setting if PayPal Account Optional is turned on in your merchant account. Passing Sole has no effect if PayPal Account Optional is turned off in your account
Hope this helps.