the abandonedarmies.abel.pbo (or something else along those lines) would go in the Missions folder under Codemasters/OperationFlashpoint, and editorupgrade102.pbo would go into the Addons folder.
For more detailed instructions on the instalation check the readMe.txt provided with the mission, and Mikero's website, which has a section on mod folders for easy addon management.
Mikero's SiteHave fun with the mission, it's will almost definately be the best one you have ever played. Although, if you are new to the game, I would recommend playing through the Resistance campaign, aswell as the third party Rettalliation campaign and a few other user missions before tasting this masterpiece. You will appreciate it all the more if you have built your OFP skills to a relatvely high level before attempting Abandoned Armies.