There are several ways to install an extension:
In the Extensions area of the Start Page, you can see a list of extensions available for your GeneXus version. Each listed extension has an Install link, which allows you to automatically download and install the extension.
Using the Extensions Manager (you can access to it by selecting the option: Tools / Extensions Manager in the GeneXus menu), you can see all the installed extensions. An Add button in the upper right corner shows you a list of available extensions from which you may choose any one to automatically download and install.
You can manually download and install any GXextension by placing its corresponding .dll files in the Packages folder of your GeneXus installation path. For example: "C:\Program Files\ARTech\GeneXus\GeneXusX\Packages".
And finally running GeneXus with the /install parameter. For example: "C:\Program Files\ARTech\GeneXus\GeneXusX\genexus /install".
This does not actually start GeneXus, but it makes it scan the Packages folder and install any extension it finds in it. Next time you start GeneXus, you should find the new extension listed in the Extensions Manager.