Uh oh. Thanks for trying. And here I was thinking lua was cross-platform and all...

After a short reading up on it, it seems popen is supported on mac, but only if it's compiled into your lua (or in this case, Löve engine). You're using löve 0.8.0? If so they just forgot to compile it in?
I'm trying to think of a workaround... I can workaround this specific error, but later on I do need popen to list all files in a directory.