Networking on a Mac is usually hassle free - it just works.
What I would do is this:-
Go into System Preferences > Network and delete all the network connections except your WiFi.
(to do this, click the connection, then click '-' at the bottom)
Now, add a new connection by pressing the + button. Choose Ethernet, and name it Duet
Next, where it says 'Configure IPv4', change it to Manually.
Enter 192.168.2.10 in to the IP address box and 255.255.255.0 into the Subnet Mask Box. Leave the gateway blank.
Now, click the cog at the bottom and select 'Set Service Order' and make sure your WiFi connection is above the Duet one. Most versions of OS X will be default prefer a wired connection to a wireless one, so if you don't do this, your WiFi won't work whenever you're plugged into the Duet..
Next, open a terminal window and perform a sanity check by pinging 192.168.2.10 - it should work.
Now, try pinging 192.168.2.20 and see what happens. If it doesn't work, then I suspect the problem is one of three things:-
1) Ethernet cable
2) Ethernet port on Duet
3) Something wrong with the SD card
To be honest 3) is by far the most likely I'd say.
