i386 (Intel) CPU, running OSX 10.6 or higher
Download Cn3D 4.3.1 here:
Note for Gatekeeper-enabled Macs (OSX 10.7.5 and up):
Simply download the archive via this link, and extract it into a folder on your system, wherever you normally put application software, using your favorite zip archive application.
Once done, see the instructions for configuring your browser.
Solving the "unidentified developer" problem:
If you encounter the message "Cn3D can't be opened because it is from an unidentified developer" when installing or running Cn3D, it is very likely the OSX "Gatekeeper" software is the cause. Apple has provided instructions you can follow to exempt Cn3D from Gatekeeper at http://support.apple.com/kb/ht5290, specifically, in the section "How to open an app from a unidentified developer and exempt it from Gatekeeper."