It looks like you are missing two parameters:
<input type="hidden" name="item_name" value="Itemname" />
<input type="hidden" name="amount" value="2" />
the item name and the item price. once you add those to your form, the button should work correctly. Those fields should have been created when you clicked OK. One thing you can try is reentering the wizard by clicking on the paypal button in design view in DW and then open the Properties window (Window -> Properties) and click the edit button and make sure to specify a total and a name for the product.