3com router config

I have a satellite site connected via frame relay and 3com routers…. I have recently had to recreate the site from scratch, and have had the opportunity to configure these routers with static routes for the first time… easy so far… i can access all my own private subnets with the routes i have put in, but i need to define a default route that will be used in the event of a packet not being destined for my network (i.e. one for the internet)… any idea how to add this? something that saysall packets not matching another route go to : 192.168.90.2 (my internet adsl router)??