The Mac OS X client will unpack correctly with gunzip on Mac OS X 10.2 (jaguar) or 10.3 (panther) as long as you type the command within Terminal. Stuffit 7.x or newer will work under the Finder in either OS X or OS 9, but I'd recommend using 'gunzip' or 'gzip -d' within Terminal instead.
However, the two main browsers on OS X (IE 5.2.x and Safari 1.x) will automatically unpack downloads by default, so your work may already be done.
If you use IE, the boinc client will download and automatically unpack leaving two files:
#2 is the unpacked program ready-to-run. You can just start Terminal and run boinc.
If you use Safari, the boinc client will download and automatically unpack, leaving a single file:
Here's what you have to do to fix the Safari download (apologies if you already know how to do this):