Can you send a link where i can see the problem?
It could be that a php error is occurring, add the following code to your pages to turn error reporting on:
I'll PM you a link and login info as I have this set up on my sites back-end.
For the payment gateway, I selected paypal website payments standard, and entered the correct business credentials (secure merchant account ID) from my paypal profile page.
I assume I should add that code to line 1?