Please see this guide on how to program an Xbee module using only a serial terminal: ladyada.net/make/xbee/configure.html
This guide should be cross platform. The only consideration to have is making sure the serial terminal program does not send unwanted carriage returns or newline characters.
We have not tried to do it on a MAC, I personally configured XBee modules under Windows and Linux. The only difference when using a Mac is the serial terminal program, there is nothing special about MACs in this realm. We suggest looking more into the commends you are sensing and the expected reply rather than your particular OS version.
