https://trisquel.info/en/wiki/macbook
In my experience, Trisquel's ISOs don't boot directly on the Apple firmware. So, you will probably need to install something like Ubuntu/Lubuntu first (using their amd64+mac ISO); then flash Libreboot; then you can install Trisquel. They're very nice laptops and they work great with Trisquel - I use one myself :-)
