When I try to use it to capture a movie, the "Mac Audio Track" option is grayed, and can't be turned on. This makes it impossible to record audio. Anyone have any idea why this is happening?

Hello,
I would be more than happy to help with the problem. In the future you may find the online FAQs are a more convenient source of answers to questions that you may have. You can find them at:

http://www.ambrosiasw.com/support/faqs/

Be certain that you are checking the "Mac audio track" checkbox in the Movie Settings palette BEFORE you being your recording. If that "Mac audio track" option is grayed out, be sure you are running the latest version of SnapzProX 2. You can find it at:

http://www.ambrosiasw.com/utilities/snapzprox/addons.html

1. Invoke Snapz and press command-Q to quit Snapz
2. Remove the Snapz preference file "com.ambrosiasw.snapz.plist", which is located in the Home/Library/Preferences folder
3. Remove the the "AmbrosiaAudioSupport.kext"  from System/Library/Extensions folder. You may be asked for the admin username and password. If the file is missing, don't worry.
4. Run the SnapzProX 2 installer again. This will replace that file, which is essential to the audio recording.