1.) If the mission is a pbo file (an extension of .<islandname>.pbo) then you will have to grab a pbo extractor such as Amalfi's one, which can be found here:
http://ofp.gamezone.cz/utilities/pbo_decryptor15.zip. Chosing your mission pbo in this program and running the utiliy will produce a folder (in the same directory as the .pbo you just decrypted) with the extension (if folders can be said to have an extension) of .islandname, where islandname is something like eden, noe, cain and so on.
2.) Cut this folder, then look in your ofp directory users/<yourname>/missions (i think, or it may be called usermissions, cant check right at the mo.) Dump the folder in there, and boot up ofp and the editor. Voila!
If the mission was a folder to begin with, simply follow step 2.