I fear I'm not very familiar with mac's architecture or MBR, so I probably wont be of much help on the specifics, but are you dual booting or is this the only Operating System on the Hard Drive? Does the hard drive in question have priority on the BIOS's hardware boot order?
If everything is alright on the BIOS side, try booting to the operating system after installation using a boot disk and make sure the configuration is correct; I found information about configuring the boot loader and links to at this website ( http://www.askdavetaylor.com/how_do_i_dual_boot_ubuntu_linux_mac_os_x.html
) which says that Ubuntu (which I was previously unfamiliar with) is a Debian derivative and thus uses yaboot, which would surely have all the necessary documentation on their site.
If you keep having problems try reinstalling the boot loader, I'm sure there's a download at yaboot's site to get the most recent release, or if that fails even try another boot loader.