You probably already have Expandr as it ship with most macs. Look in Applications/Utiltieis for it. If there - launch. Right click or Control Click the icon for the Agfa drivr and go to the Open With on the pop up menu. Select Stuffit Expander and see if it opens.
Normally, files with the .SIT extension require Expander to open. If you download has the .DMG (disk image) estension and it does not open, something could be wrong with that file.
Using OSX 10.3.9 on a G4