Hi GFA-SkyRanger,
Are you referring to installing an Engine plugin into the binary version of the Engine that is installed by the Launcher, or are you trying to install a plugin into an Engine that you have built from source code? For the former, you will only be able to install plugins that are available through the Marketplace on the Launcher, since you will not be able to rebuild the Engine once the plugin is in place.
For the latter, you should place the source code for the plugin into a folder in the Engine/Plugins folder. Once that is done, run GenerateProjectFiles.bat, then open the UE4.sln file and build the Engine again.
If you are trying to add a plugin to your project, you will need to create a Plugins folder in the project’s root folder and put the source code for the plugin there. Then right-click the .uproject file and select the Generate Visual Studio project files option, open the solution in Visual Studio, and build the project.