I use a Linksys router on a cable modem with Roadrunner. The setup is done entirely with a browser.
What you need to do is use the router's browser interface to configure two things:
1. What you want your internal network to use to access Earthlink.
2. How the router connects to Earthlink.
Access the router with your browser. I think the default IP address is 192.168.1.1, and the default passwd is "admin", and no login name. (You should change the passwd soon!)
Click the Setup tab. Here you specify your connection to Earthlink and the address of the router.
Under where it says "LAN IP Address," you can set the IP address of the router itself. This is the address you will use for "Router" setting in your Mac (& PC) Network control panel. It's okay with the default, which is probably 192.168.1.1. Set the subnet mask to 255.255.255.0.
Under "WAN IP Address," you probably should set this to "Obtain an IP Address Automatically" (unless Earthlink has given you a static IP address, which is not likely).
Under "Login," select "PPPoE" unless Earthlink tells you otherwise. Enter your User Name and passwd if needed.
Leave the "Keep Alive" setting alone unless Earthlink tells you to change it.
Click "Apply" at the bottom of the page.
Click the "Status" tab. Everything should show as you entered it for the LAN stuff, and the WAN IP address should be whatever Earthlink assigned you. (This IP address may change often.)
Take note of the DNS server(s) IP address(es) under WAN. You *may* have to enter these addresses in your Mac's Network control panel.
Now, go to your *Mac's* Network control panel. Enter the following, given that the router's IP address are 192.168.1.1 and it's subnet mask is 255.255.255.0:
IP address: 192.168.1.2
Subnet mask: 255.255.255.0
DNS servers (if needed): Enter whatever was reported under WAN on the "Status" tab of the router config page.
That should get you started. If you need more help, give me shout.