To install SAM plugin extract the zip file into your X-Plane root directory and overwrite if prompted. All files will be placed automatically to the correct directories.
For developers only
If you would like to place objects from the SAM Library in WED you will notice that the library is not available in the selection list. Therefore, one of the following steps is required:
Copy the SAM_Library folder from resources/plugins/SAM/lib into your custom scenery directory. Remember to do this after each update
Use a LinkShell Junction to drop a static link to the SAM/lib folder into your custom directory (tutorial)