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