You need to forward port 80 (http) on the linksys router to the address of the SME box. As for suggestions of dumping the linksys router, there's lots of reasons against that. I actually have a few routers in front of my SME box.
First, I have an ADSL modem, then a netgear router with backup 56k dialup, which forwards everything onto an Asus WL500G wireless router behind it. I have another WL500G in another room, acting as an access point, and my SME box behind that. The first WL500G forwards all the ports the SME box needs. Then I have my internal network behind the SME box.
The result being I can give the netgear router back without reconfiguring my network (it would have been cleaner to set both the asus routers to be access points only, but I didn't always have the netgear, and don't trust it yet), and any wireless clients that connect need a vpn to get to my internal network, so if my wireless network's compromised, someone might steal my bandwidth, but won't get to my data.
In your situation, I'd keep the linksys router (they're great, as long as you don't try to use them as a non-NAT router, as they don't let you have enough firewall rules), and put the SME box behind it, and any pc's you have behind that. Use a VPN if you want to connect wireless clients to the internal network. If your SME box is in Server and Gateway mode, you can tell it to get it's IP using DHCP, and set the IP statically on the linksys router by giving it the SME box's MAC address.
You can find the MAC address of the SME box by typing 'ifconfig', or possibly in the linksys router's arp table.
If you don't have a static ip then you'll need to setup dyndns or similar.
Hope this makes sense.