A plugin loader is required to run any of the available plugins, since CMSimple by itself is not able to handle plugins. Addons on the other hand usually do not need a plugin loader, if not specifically stated.
The best choice in the moment is Plugin Loader 2. It should work not only with all newer plugins, but also with the old ones (Geniz, Svarrer). However, everybody has to test it in their own environment.
Plugin Loader 3 is just a kind of working hypothesis. It will not work with any of the existing plugins.
In order to install a plugin loader, it is just copied into the /plugins folder. In case such a folder does not exist, it has to be created in the CMSimple root.
Afterwards you have to tell CMSimple in its configurations which folder the plugin loader resides in.
For a more detailed explanation of installation see: plugins
Recommended PluginLoader
Pluginloader_Archive