what you need to do is insert you mac os x disc, and restart the computer. when you hear the chime, hold option until u see the mouse and hard drives come up. now click on mac os x cd. click ur language and then the arrow, now when the new installation part loads, plug in ur backup, and go to the top menubar and hit utilities. now hit restore from a backup. select the external hard drive and whatever partition its on, then click continue. now be careful here, especially if you have more than just macintosh hd plugged in. click macintosh hd, and double check. then click continue. it will restore it, then it will restart, when it restarts eject the cd. and ur finished.
Since this is a mac, and you do have a backup, only if you dont have the cd, go to the apple store and ask to borrow their installation cd's that they keep on external hd's so u can restore from a backup. im sure if you show them ur mac and ur backup they will gladly allow you to do so.
good luck! and merry christmas!
