Hmmm...

You could try this, while following these instructions.

1. download ffmpegX
2. open .dmg package
3. control-click on ffmpegX and select "Show Package Contents"
4. find Contents/Resources/ffmpeg
5. copy ffmpeg to somewhere like /usr/local/bin
6. set the path to the ffmpeg binary in Gallery (e.g., /usr/local/bin/ffmpeg)

You could also try putting the binary where I previously mentioned.
You must also place the files I mentioned earlier:
pthreadGC2
ffplay