OK, one more thing just occured to me. You have an airport base station right, and I am guessing the mac you are using is a laptop or something else with a wireless card that would normally get internet access through the airport. Is that right? If so try this...

The airport base station will do DHCP by default so.
Go to your TCP/IP control panel on your mac and set it to
Connect via: Airport Card
Configure: using DHCP Server
Close the control panel
Using your cross over ethernet cable connect your empeg to the Airport base station.
At this point, the empeg and the Mac should be able to get all needed info from the airport

At this point jemplode on your mac should be able to connect to the empeg with no additional configuration.

-Mike