If its on an official apple system, plug the drive into the mac, boot the mac from the DVD (hold down the C key when you hear the chime noise as it powers up) and then point it to the external drive when asked where to install to.
No, to boot on a PC it needs to have a different bootloader and some drivers to emulate the Mac EFI and SMC hardware at a minimum. If you booted a distro installer and installed to an external device it might work, as might using nawcom's modcd to install a retail disc, but I have not attempted these myself. Bare in mind that if it works, it would likely only work on the system you installed it on and those with very similar hardware.
I have a lot of experience doing this if you need help. What kind of system are you trying to run it on? Start here: -search for a tool called iBoot and download it -burn the iso to cd -boot from the iboot cd -when iboot loads, eject cd -insert your mac installation media -let the disc spin up and press F5 -this should get you booting started -if this doesn't get you running the installation cd, hit me back and I'll help you figure out what boot strings will work