Are you testing by entering a credit card? Or by using The express checkout option?
If you are using the express checkout option, these are not the correct bindings. When adding the Bindings to the Bindings Pane, select the Authorize Express Checkout and the Process Express Checkout options in the gateway list. Ge the full request and Full response from both.
If you are testing by entering a credit card and not getting a response back, this means that paypal is not excepting the API credentials that entered, they are not sending a response back, because you are not passing the initial authentication. double check that the values you entered are valid api credentials.