Ubuntu installer will not see os x as another os installed on the ssd drive

I have a MacBook Pro (Retina, 15-inch, Late 2013) with El Capitan 10.11.6 and i also have rEFInd is installed. When I boot up the rEFInd boot menu comes up and I have mac, boot camp and 4 linux options when there should only be one linux option, I create the bootable usb on mac as instructed by the ubuntu site using unetbootin. when I try to install ubuntu from the bootable usb, it does not detect os x as another os so I do not have the option to install ubuntu alongside os x. I would like to know why the installer does not detect os x.