The zip file for a experimental Chimera feature contains a directory. Put this directory in your Chimera distribution in the "share" directory. The location of this directory depends on where you installed Chimera. Here are locations for standard installation locations:
To display this folder on the Mac hold the ctrl key and click on the Chimera icon, then choose Show Package Contents from the menu that appears. From there proceed to Resources/share. Another way to see this folder on the Mac is to use Finder menu entry Go / Go to Folder... and type the full path to the folder.
Restart Chimera and the experimental feature will be available, usually adding a new command or menu entry. See the experimental feature detailed description for use instructions.
If the directory provided by the Chimera experimental feature zip file already exists in your Chimera share directory, then you need to replace the copy in Chimera. Do not simply rename the original directory leaving it in share because Chimera will still find it and this can cause errors because two copies of the same tool will be installed. It is a good idea to keep the original directory in case the new experimental code has bugs. One method is to make a subdirectory within share, for example one named original, and move the original code there for safe keeping. Or the original code can be moved to the trash where it can later be recovered.
When you update to a new Chimera release it will not have the experimental features you installed in your older release. It is tedious to reinstall the desired features. An alternate method of installing experimental code can be used to minimize the work when you update Chimera. Instead of placing the new experimental feature directory within the Chimera distribution, put them in a separate directory, for example, one called chimera-plugins which can be anywhere on your computer. Then use Chimera menu entry Favorites / Preferences, select category Tools, and press the Add... button below the Locations panel at the bottom of the Preferences dialog. Select the new chimera-plugins directory. This makes Chimera search the chimera-plugins directory for new tools. Finally press the Save button on the Preferences dialog so that Chimera will search this directory every time you start it. The path to your plugins is saved in your Chimera preferences file and will continue to be searched even when you install a new Chimera release.