Message box? In Mac OS X you generally don't use a message box, or when you do, they fold down from the top of the window. Assuming you have a window already, this is the way you should do things. You'll probably have better luck looking in that direction. Maybe somebody will come along with a slicker answer than that.

I've done a MessageBox function for mac, you will find the code in my blog there you will find a function that create a modal message box and returns the selected value depending on the pressed button.
The code, with few more lines of code will be a complete equivalent of window's MessageBox function