I think you have to have a supported machine for the temperature report to work...however, I couldn't find a list on their website. I have a MDD G4 and the temperature portion is not available on my machine. I know they require you to install the fan and temp extension on the intel machines.
the newest version of istat pro works on all intel macs, temperature and even the fans. http://www.islayer.com/index.php?op=item&id=7
the apple website doesnt have the newest version, they are actually 2 releases off.