I'm a long-time Mac user, and unfortunately, I haven't found any machine embroidery programs for a Mac. However, since you have a MacBook, there is a work-around. The newer Macs use a dual-core processor, which means you can partition the hard drive and load and run Windows and Windows programs on it using a program from Apple called Boot Camp (see the link below to find out about it). You need a copy of Windows XP, and you have to restart your computer to move between the systems, but it means you can download the one of the free conversion programs like Wilcom Truesizer, or you can run Embird to edit designs.

Here is a link that might be useful: Apple Boot Camp