Missions written for recent versions of Aces High are easier than those written for older versions. (At first, you'll want to stick with missions that have been written during the past couple of months. With experience, you'll learn how to pick older ones that are likely to work.)
As of this writing in October of 2010 for Aces High Version 2.21.x, the procedure is:
-- Go online and find a mission file somewhere. Often these missions are stored on web pages that describe the mission and may even include a movie showing what it looks like. Each mission is written for a specific terrain.
-- The associated web page should let you know if you need a special terrain. If you don't have that terrain, you'll have to download it too. Terrain file names also end in ".res". You will need to store the associated terrain file in the "ahiiterr" folder of your recent Aces High installation. At that point, you can test the terrain by flying around in it offline.
-- Once you are sure you have the terrain for which the mission was written, you're ready to download the mission itself. Mission file names also end in ".res". You will need to store the associated mission file in the "missres" folder of your recent Aces High installation.
-- Then, from the startup menu of Aces High, be sure to designate the associated terrain. Select "Offline Practice". Then, from the "Tower", select "Offline Missions". A list should pop up giving you the option to select all available missions that you've installed for that terrain. If you don't see that option, then something went wrong when you tried to download and store the .res file in your "missres" folder.
Once you see the new mission name(s) in the available missions list, left-click on it. A second colum will list the available flights within the mission. Choose between the Bish or Knights options. Your offline mission will commence immediately.
I wrote a very simple mission for people that just want to get started with all of this. It uses the standard, well-known "ndisles" terrain, so you won't need to download a terrain for it. Versions are available for old and new editions of Aces High. Choose the one that fits your situation. This link will take you to a description with movies you can watch, detailing the exact, suggested method of installing it:
http://techvideoreview.com/FlightSimMovies/AcesHigh/Ah2Missions/PeabodySimple/PeabodySimpleMissionPage01Full.htm
