You'll have to manually partiton your drive first.
When installing YDL the installer will ask you how to install. YDL installs on an Ext2 or 3 filesystem, so you'll have to "assign" a partition first (between 4 - 10 GB's should do).
Linux also requires a "swap space". This is a pice of HD assigned for Virtual Memory.
Also included is yaboot (a loader so you can choose which OS to boot from)....
I have an iBook 600 MHz, 256 MB RAM, 20 GB disk, with OS X and YDL 3.0.
HD partitioned as follows (in correct order):
6 GB Linux: (3 partitions)
-32 MB "Yaboot" (loader)
-256 MB "swap space" (VM)
-5.5 GB "/" (Ext3 filesystem = root, space to install Linux)
6 GB Mac OS X:
-6 GB HFS+
6 GB Data
-6 GB HFS+
I install the 32 MB loader (yaboot) on the FIRST partition of the drive, so that PRAM reset is sufficient to tell the Firmware to use this loader as the startup disk
Practically:
Install Mac OS X, using the startup CD's Disk Utility to create the partitions first, reserving the FIRST 6 (or so) GB's for Linux. Just format it as Mac OS Extended (HFS+), as the Linux installer will reformat that volume.
Install Mac OS X on the SECOND partition.
Once finished with that, install YDL using the FIRST partition to create the loader's space FIRST, the SWAP second, leaving the rest of your predefined Linux space for the 'root' (/). Install Linux on that, and let the installer guide you.
Have fun.