Getting started:
1# Import all in the Platformer Engine (1.4.5) extension in an empty project.
2# Import all in the MultiplayerAddToPlatformerEngine.
3# Go to "Included Files" in the project and right click on CloseProjectAndRun.bat and click "Open in Explorer"
4# Close Game maker studio (Close your project)
5# Run the CloseProjectAndRun.bat (This will replace some files in the Platformer Engine extension)
6# Start Game maker studio and load your project and run it.

If you have done above and now downloaded an updated version of MultiplayerAddToPlatformEngine:
1# Close Game maker studio (Close your project)
2# Run the Update.bat
3# Start Game maker studio and load your project and run it.