Cn3D macromolecular structure viewer
Downloading Cn3D for Macintosh
  System requirements:
i386 (Intel) CPU, running OSX 10.6 or higher
Download Cn3D 4.3.1 here:

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.
Note for Gatekeeper-enabled Macs (OSX 10.7.5 and up):
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, specifically, in the section "How to open an app from a unidentified developer and exempt it from Gatekeeper."
The previous version, Cn3D 4.3, also continues to be available:

Revised 20 September 2016