I have a MAC too. I haven't tried to take a test on my mac yet but I found the following info on the NBME website. Goodluck.
System Requirements
Microsoft® Internet Explorer 5.1 or higher or Netscape® 7.0 or higher. Internet Explorer is recommended.
Windows® 98, 2000, ME, NT, and XP.
Color monitor capable of displaying 800 x 600 with 32 bit color. For best results, use a 17 or larger monitor with 800 x 600 resolution.
Your computer must accept cookies
Popup window software must be disabled
It is highly recommended that you use a Windows-based machine. Although some examinees choose to take the self-assessments on a MAC, and the software may run acceptably, please note that we do not offer support for the MAC platform at this time.