Use this Program "CoreDuoTemp" at this link http://macbricol.free.fr/coreduotemp/. OSX does not have a similier program built into it from what I can see.
If you download coreduo temp, and have istat pro widget, then the temp function will start to work. (If it didnt before, like on the MBP). iStat pro does everything, and makes it easy to understand.