what's on the dvd?
is it a dvd-video (eg. a movie) or is it a data dvd with files on it (for example, a collection of pictures or music).
If it's the first, you don't need iDVD (which is used for making your own homevideo dvd's) but you need to use DVD player (should be in your applications folder)
If it's a non-original (burned) dvd, changes are you've got a problem with dvd-r/dvd+r media: Some older powerbooks only accept dvdMINUSr media. If your disc is burned at dvdPLUSr media, you're out of luck. if you post the speed of your mac (apple menu --> about this mac) we can determine this.