First off, uninstall the one you downloaded, and get this instead:
http://downloads.sourceforge.net/fretsonfire/FretsOnFire-1.2.451-macosx.zip?use_mirror=osdn
If you still got the issue, know this is a big thing with FoF. I had a friend go through the same thing. Here's what you essentially have to do.
1. Download version 1.2.451 of FoF, uncompress it, and stick it in your Applications folder.
2. Right click the Frets on Fire.app that you put in your Applications folder and choose "Show Package Contents".
3. Now download version 1.1.324 of Frets on Fire, and uncompress it to your Desktop. The link to download is below:
http://downloads.sourceforge.net/fretsonfire/FretsOnFire-1.1.324-macosx.zip?modtime=1166696687&big_mirror=0
4. Right click the Frets On Fire.app that you just uncompressed on your Desktop, and again choose "Show Package Contents".
5. In this 1.1.324 package contents folder on your Desktop, open "Contents/Resources/lib", copy the Python 2.4 folder, and paste it in the exact same location in the 1.2.451 package you first downloaded. (Again, it's "Contents/Resources/lib"). If it asks to replace, do so.
6. Open up your Macintosh HD. Browse to Library/Fonts.
7. Copy the "Chalkboard.ttf" file. Bring up your 1.2.451 package folder again. Browse to "Contents/Resources". Open up the Data folder.
8. You should see a "defualt.ttf" file. Rename it to "default.ttf.bak". If prompted, select "Use .bak".
9. Paste that "Chalkboard.ttf" file you copied earlier.
10. All done. Start the game normally.
http://fretsonfire.sourceforge.net/documentation/faq/#macosx
Q: The Mac OS X version only shows a blank screen or crashes.
A: The Mac OS X version is known to be buggy, especially on PPC machines. Unfortunately, as none of us inthe development team have Macs, we can't really improve the situation. This thread in the forums may be helpful. If you're a Mac developer willing to help out with this, please get in touch.