Installing Openplanet is straight forward. First, download the latest version and run the installer.
During the setup, you will be asked to locate your install path for ManiaPlanet. Make sure you enter the exact directory where
ManiaPlanet.exe resides. (If you're installing the Trackmania version of Openplanet, you're looking for
Trackmania.exe, and for the Turbo version it's
TrackManiaTurbo.exe.) The setup will do its best to figure out the path for you, but it could be wrong.
Once installed, simply start ManiaPlanet like normal, and you should see "Press F3 for Openplanet menu" in the top-left corner. If the overlay doesn't work, check out the troubleshooting guide.