The Plugin Manager dialog is accessible from the "Tools" menu in the main menu bar.
Using the Plugin Manager, it is possible to see which plugins are active, as well as load and unload plugins.
It is also possible to manually load plugins, by using the "Browse" button and choosing a python file.
Note: If the plugin is a module (a directory), it can be loaded by choosing the "__init__.py" file inside the module.
The "Refresh" button can be used to update the plugin list. This is useful when plugins are loaded using the Python API.