I own a MacBook Pro running Mountain Lion. I would like to dual-boot my Mac OS with Ubuntu. When I download the file and try to open it, it reads "No mountable file systems". I have downloaded both the 64 and 32 bit software. Can someone help me through this process?
You need to set up the Mac so that there is a valid file system for Ubiquity to find and start installing to. Have a look at the Mac installation wiki and go from there as they have done a wonderful job already.