1. Boot your Mac OS X install dvd
2. Once the installer is running, go to the Utilities menu and open Terminal
3. Determine which disk your MacOSX partition is on
Type diskutil list
Verify which disk number holds your partition (disk0, disk1, etc.)
4. Start using Fdisk
Assuming the MacOSX disk is the first disk ("disk0"), then
type fdisk -e /dev/rdisk0 <== use "rdisk" with your disk number here !!
Ignore the error "fdisk: could not open MBR file ..."
5. Determine which partition for MacOSX needs to be set "Active"
Type p
Verify which partition is for MacOSX (1, 2, 3, etc.)
6. Set the partition "Active"
Assuming it is partition 1, then
type f 1 <== use your partition number here !!
7. Save and exit
Type write
Type y (yes you are sure)
Type exit (to quit)
8. Remove the install DVD and reboot