Not able to boot into Ubuntu, after installing on separate partition, boot partition?

Hi Ubuntu Community,

Im fairly new to this but i was just wondering how I would go about holding down the alt key on my mac to launch my newly installed Ubuntu OS or Mac OS X.
I've installed the OS to a separate partition on my 500Gb HDD and managed to get it running(after installation), create a user and create a couple of documents plus some setting changes.I shutdown the computer and now can't seem to get the option to boot back into the installed version of Ubuntu.
Any help would be greatly appreciated....

Heath

I was hoping i could start up Mac OS X or Ubuntu by defaulting to Mac HD or holding alt/option key to choose Ubuntu or recovery HD. I made the advised 2 separate partitions one 100Gb ext4 and one 10Gb swap then installed the boot loader on the 100Gb partition (is that correct), i wasn't sure but it seemed logical....

PLEASE RESPOND, I have little experience on this site and with Ubuntu installation....I wasn't given the install alongside option either so it proved rather difficult

MASTER BOOT LOADER????????MasterBootLoader