Instructions
Download the desired mod files. Do not extract them.
Open "My Computer" and open the folder where "TrackMania" is installed. If this was not modified during install, it will be in "C:\Program Files\TrackMania Nations."
Open the "GameData" folder, the "Skins" folder and then the "Stadium" folder. Open the "Mod" folder. If there is no "Mod" folder, right-click anywhere within the "Stadium" folder and select the option to create a new folder. Label it "Mod" and open it.
Copy the downloaded mod files into the "Mod" folder.
Launch "TrackMania Nations" and open the track editor. Hold the "Ctrl" button and click on the map. Select your desired mod from the list. Save the track to apply the mod.