I always believed that I would need to configure the gateway provided by the ISP somewhere on the netscreen in order to make this work. I will try just adding the MIP using an IP from the secondary block and see if that gives me the result I ant.
As far as asking the ISP to make sure the IPs are routed to my modem, I am pretty sure they will not help me since this is a residential account and not a business account. So I have my fingers crossed that it works and I don't have to tell the customer to upgrade any futher. Thanks guys.