If you're using the excellent wiki Installation/OldWorldMacs but don't know how to use miBoot or BootX, this page will hopefully give you ample instructions. Please be aware that BootX is only needed for Old World Macs and requires that a functional Mac OS (not OS X) system is installed on the first partition of the hard drive.
First download and install BootX from http://penguinppc.org/~benh/BootX_1.2.2.sit. Copy the "Linux Kernels" folder to your System Folder, the BootX extension to your Extensions folder, and the BootX App to your Control Panels folder (BootX App can be copied anywhere, but this is a good place to put it).
You then need to copy the Linux kernel from the Ubuntu installation CD to the Linux Kernels folder in your System folder. The kernel is located at install:powerpc:vmlinux. Now open BootX App. The kernel selected by default should be the one you copied (vmlinux) but you'll need to specify a RAM disk (initrd.gz) so select Options... and check "Use Specified RAM Disk". If you're not prompted to browse to a RAM disk image then hit the choose button and browse to the initrd.gz file at install:powerpc on the install cd.
You can now press the Linux button to boot into the installation system and follow the Installation/OldWorldMacs instructions from there. Please be aware that you will have to copy the Linux kernel and RAM disk image from /boot on your new Ubuntu installation to the "Linux Kernels" folder in the Macintosh System Folder before you can boot into the newly installed Ubuntu system.
For more details on using BootX go to http://penguinppc.org/bootloaders/bootx/.