There's no official Meld packaged app for OSX, but reading this blog post led me to this package. Unfortunately I could get the later versions to work, but the origin alpha (here) worked for me.

Alex Kras has provided a script to enable it to work from the command line, but I was sure there was a simpler way to open it, and after some messing around, it seems you can use OSX's open command, so you can set the following in your .gitconfig to open Meld automatically using git mergetool.