Mac OS X 10.4 - This is a PowerPC and you'll need a special version of SignCut.
Mac OS X 10.5 - This version is a bit tricky, since it can be either a PowerPC or Intel. To find out which one you have, please go to the Apple menu and choose About This Mac. Here you want to take a look at the line that says Processor. It can for instance says "Processor 1.67 GHz PowerPC G4" which means that you have a PowerPC, or it can say "Processor 1.83 GHz Intel Core 2 Duo" which means that you have a Intel CPU.
SignCut for PowerPC can be downloaded here.
SignCut for Intel Macs with 10.5 can be downloaded here.
Keep in mind that this is not the same version that is on the homepage since that version of SignCut has a few updates that OS X 10.5 in unable to manage.