Marketpress checkout not going to paypal express checkout

I've enabled the paypal sandbox on the site to test paypal express checkout, this is the only gateway selected.
I've entered the api username, api password and signature into the payments setting page and set the paypal mode to sandbox.

click product image on homepage
click buy now on single product page
click checkout now
enter shipping information
click continue checkout

This is where I'd expect to be redirected to paypal but instead I'm getting the order confirmation page.