OK. Use of offline missions requires familiarity with two concepts:
1 of 2: Files associated with "Missions", and
2 of 2: Files associated with "Terrains".
Let's take #2 first:
Every offline Mission ever written was created with dependency on a specific "Terrain Map". There are a lot of these maps. Most were created by individual users, and you don't get them automatically when you download the main Aces High software; you have to find and download them individually as you need them. Once they are installed, they remain in your system through most of the routine updates. If you don't have that map, you can't run the associated mission, and you'll have trouble seeing it even if the mission files are properly stored.
You can see a detailed example of a "Terrain", with detailed instructions on downloading and storing the associated file here:
http://techvideoreview.com/FlightSimMovies/AcesHigh/Ah2Terrains/Ah2SolomonIslandsTerrain.htmNow it gets uglier as we discuss item #1:
At the time of this writing in May 2010, the community is suffering with a recently updated version of the Aces High software that crashes a lot when complex offline missions are used. However, some simpler missions are still compatible. You can see a detailed example of a simple "Mission" (that still works in May 2010) with detailed instructions on downloading and storing the associated file here:
http://techvideoreview.com/FlightSimMovies/AcesHigh/Ah2Missions/SolomonMelee/Ah2OfflineMissionSolomonMelee.htmBe sure to read and follow the instructions carefully; it isn't difficult, but there is a need to RIGHT-CLICK at one point, and if you LEFT-CLICK instead, you're going to have trouble. Also, certain well-known web browsers will rename the required file as they store it, without telling you, rendering it incompatible, so you'll have to rename it afterward using the instructions found at the above link.
If you are using an OLDER version of Aces High (Version 2.17Patch3 for example), then you can still fly the more complex offline missions, which may in turn require installation of a lot more mission-related files. You can get details on that task, with very specific, sophisticated examples, here:
http://techvideoreview.com/FlightSimMovies/AcesHigh/Ah2Missions/Ah2HollandiaCampaign.htmThat mission is about as complex as any ever written, so the documented instructions can serve as a kind of a blueprint that should work for any other mission, even if it is also very complex.
For a general introduction to offline missions, explore:
http://TechVideoReview.com/FlightSimMovies the follow these prominent links:
Aces High -> Offline Missions
I hope this helps!