[ubuntu] Mac dual boot loader install

I have an iMac with a large mac partition, a small windows partition and -- now -- a medium Ubuntu 14.04 partition. Everything went fine with USB install using Mac Linux USB Loader until the end of the download. After I restarted with USB in and help down the alt/option key, I installed Ubuntu, but at the end it said that it could not install a boot loader and that this was necessary to be able to boot up the program.

I have rEFit installed, and I know to hold down the alt/option key on restart, but indeed Ubuntu could not be found. How would I go about manually installing a dual (or tri) boot loader for iMac 10.8.5?

Thanks in advance,

Dylan