http://askubuntu.com/questions/688632/dual-boot-mac-el-captain-along-wi…
itt van leírás, nem tudom így csináltad e.
A lényeges dolog ott van a végén:
At this point, when you reboot, rEFInd should appear and give you options to boot either OS X or Ubuntu. Chances are both will work; but if you used a separate /boot partition, you'll have to hit F2 or Insert twice rather than Enter to boot Ubuntu. In the resulting screen, you must add ro root={whatever} to tell the kernel where your root (/) filesystem is; {whatever} is a description of that location, as in /dev/sda7 or /dev/mapper/ubuntu-root. After you boot, running the mkrlconf.sh script that comes with rEFInd should obviate the need for adding the root= option.
Remélem ez segít.