Ah, Stefan. Someone needs to fix that download link for the Mac download:
-bash-2.05b$ wget
http://www.graalonline.com/downloads/Graal4.dmg.bz2
--10:50:52--
http://www.graalonline.com/downloads/Graal4.dmg.bz2
=> `Graal4.dmg.bz2'
Resolving
www.graalonline.com... done.
Connecting to
www.graalonline.com:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 7,587,421 [text/plain]
Note the 'text/plain' bit.