I haven't tested it, but you can try this:
From the terminal...
sudo port install ffmpeg
then you'd need to cd to the right folder where all of your PNG files are. e.g., something like this:
then run something like:
ffmpeg -r 5 -i "renderstem_%05d.png" -q:v 2 -r 25 mynewmovie.mp4
where 5 is the frames per second to use, and renderstem is the name you used when you did raytracing:start-movie in NetLogo.
Let me know whether this works if you try it!