How to install 10.4 on a partition on a Macbook Pro that was been upgraded to 10.6.8
You’ve made the partition, but it won’t be bootable until you install OS X on it. Boot from the install media CD holding C on boot, choosing the new partition as the destination for the install. Once OS X is installed on the partition, it will appear alongside your existing install in Startup Manager when holding alt/option on boot.