I did something similar and followed the steps as noted here:
How to Make a Wireless Router Into an Access Point | eHow
Make sure that you connect to your main router and that it can "see" the router that you are having trouble with. The biggest thing is to disable DHCP on the outside router and assign an address in the range of your main router. You can also test things by attaching a cable from the outside computer to the outside router and check connectivity that way also.