DO you have an IP set for the output port within the artnet range? Something in the 2.x.x.x or 10.x.x.x range within the apple network window in system preferences?

At the top left, click apple>system prefs. Go to the network> click the ethernet adapter that has the orange dot next to it, from the drop down labeled as configure ipv4 and set to manually, create an ip address like 2.0.0.1 and the subnet should be 255.0.0.0. Press apply at the bottom and quit system preferences. Relaunch catalyst. In the catalyst control window, the top device should have the IP address listed. And if your in the HUD and you open the DMX Artnet window, the interfaces tab should also show that ip address. Also may want to make sure that in the same window under the interface tab that you have the output universe is set to artnet broadcast 255.255.255.255.

A crossover cable shouldn't be necessary as all the intels auto negotiate to the mode needed. But you could try that. Other then that, try another ethernet cable.