If you are just downloading a mission for play, the mission .res goes in your MISSRES folder. If it comes with a terrain you don't already have, the terrain .res goes in your AHIITERR folder. Do not worry about any other folders. Offline missions worked differently in prior versions with different install directions. So, as a player, you only have to deal with the MISSRES folder & sometimes the AHIITERR folder if installing a terrain for the mission. Are you guys POSITIVE you have the Y29.res in the MISSRES folder?
EDIT: Install directions from a previous page:
Install instructions:
copy arden08s.res, arden08s.fla, arden08s.fld, and arden08s.pln to the ahiiterr directory
copy y29a.res to the missres directory...
The .fla .fld & .pln files that Soda included are files that the terrain uses to set up who owns which field & which planes are enabled. They go in your AHIITERR folder along with the terrain .res.