If you are trying to load a Coop mission to play, you must first select Multiplayer from the main menu, and then New from the multiplayer menu. That will start your PC as a non-dedicated server.
If you want to load the map to edit it you must first locate the file in your OFP MPmission folder and unpbo it. Unpbo is a tool you can d/l from the Editors Depot/Tools. After unpbo move the resulting folder to your OFP users/yourusername/missions. You will then be able to edit it inthe mission editor.