NOTE: I'm making a few assumptions about the way your computer is set up. Namely:
- Your ESP partition is located at disk0s1
- Download refind http://www.rodsbooks.com/refind/getting.html
- Run
install.sh --esp
- Mount your ESP partition (
mkdir /Volumes/ESP && sudo mount -t msdos /dev/disk0s1 /Volumes/ESP/
)