Hi JamesBond,
The code you are writing will run on your Galileo board, and the ifconfig command that will be executed in the second system() line will set the configuration of the Gallileo board, not your MacBook Air.
The eth0 is the wired ethernet connection on the Galileo board, and the IP address you write afterwards is the IP address that the Galileo board will get, so it should be different from what your MacBook Air has on it's ethernet adapter, otherwise you will have a conflict. If you are just connecting the two with a cable, all you need to worry about is that they need to be unique, and only differ in the last part (e.g. .68 and .67).
When you have the Galileo board configured (should be done shortly after completed upload), you can connect via telnet on your MacBook Air using the telnet command (type something like "telnet 169.254.10.67 23" in the Terminal.app). 23 is the port number for telnet connections.
Hope this helps!
/Thomas