System requirements:

i386 (Intel) CPU, running OSX 10.5 or higher

Download Cn3D 4.3 here:

ftp://ftp.ncbi.nlm.nih.gov/cn3d/Cn3D-4.3-OSX-i386.zip

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.