Please download QCAD 18.104.22.168 from your download area. That's the file with the description:
Mac OS X 10.5, 10.6 (Leopard and Snow Leopard)
Binary for Intel only
This version fixes problems with the file dialog under some Mac OS X installations. If QCAD 22.214.171.124 does not work for you, please download and install this version instead.