You need to unpbo the file.
Under the editors depot, search for the 'PBO decryptor' by Amalfi (or choose any other unpbo program). The program will unpbo your map and you will see a folder with all of the map's components on your desktop. Place that folder into your ofp/users/your name/mission folder. Run OFP and now you can load the map in your editor. When youre finished editing, you can use 'MakePBO' (also by Amalfi) to pbo the file again.
Since youre running a multiplayer map, another thing you may want to try out is creating a folder called 'mpmissions' in your ofp/users/your name folder. If you put an unpboed map inside that folder you can run it on your server and edit it at the same time (the map name will appear in yellow at the bottom of your map list).