Help with installing Ubuntu 12.04, please?

[bbrown1]

Okay, so I have a MacBook Pro 15" (Mid 2011, 2.0Ghz i7) and I am trying to install Ubuntu 12.04 on a partition of an external hard drive (500 GB seagate GoFlex Turbo). I have 3 partions on this hard drive, 1) 200 GB for time machine backups for OS X. 2) Just 100GB for random storage. and 3) 200 GB partition to install linux on. (I made this free space using disk utility). I have installed Ubuntu 12.04 on this free space and made is the Ext 4 filesystem in the partitioning tool for linux and mounted it to "/" and installed the boot loader to the partition of my external hard drive where I am installing Ubuntu. I have also downloaded and burned rEFIt (version 0.14) to a cd. When I boot rEFIt from the CD, I am able to select the linux partition of my external hard drive, but when I try to boot, I get a black screen with the error "No bootable device". I have read other threads and saw that this may be able to be fixed by using the rEFIt partitioning tool, however when I run the rEFIt partitioning tool, I get an error that the MBR partition table was not found or something like that, so that doesn't work either. Am I doing something wrong? Is it my hardware? If anyone needs any other information, let me know