You need to install Chameleon bootloader. Use the search function above. I use release candidate 5 (RC5) but any version would likely work. There are many bootloaders available now. Be careful after bootloader installation, make sure your kernel (mine is mach_kernel legacy) and "Boot" is in place in the root directory, make sure the .kext files you need are in the /extra folder. The kernel that is working for your current boot must be in place, the installer for chameleon that I have installs a vanilla kernel automatically in root and that hoses my system after reboot.

listen to this caution: backup your kernel, boot and extra folder (if you have one) before chameleon installation. Follow the instructions in the threads you find in the search, lots of great write-ups there.