D folders usually all go into the "Decals" folder, but also worth double-checking is the decals.ini file located within the relevant skin or textures folder that resides within the aircraft folder itself - this usually gives clues or points in the right direction of where proper folder locations should be for the "D" folder.
For example, below is an entry found in one of my skins folders inside the late Albatros D.II aircraft folder:
FilenameFormat=AlbatrosD1/D/AlbD1Numbers/AD1Numbers
This indicates that I should have a folder inside my "Decals" folder called AlbatrosD1, and then inside that folder, as you can see from the hierarchy, you have a nested D folder, a numbers folder, and numbers themselves. I sometimes share the same aircraft decals folder among several aircraft, as noticeable in my example above where the late Alb. D.II calls up the Alb. D.I decals folder.
If you are flying lots of FE aircraft in FE2, you will have to tinker with some of those folder paths in your Decals.ini file, to make the decals show up. I've included some decals.ini files with a few aircraft, in my FM update pack, for aircraft that were very pesky to sort out for proper decal positioning, size of decals, etc. (such as the Siemens-Schuckert D.III by A-Team...took me a while to sort that decals.ini file - a lot of the names in many of the decals files don't point to obvious things). Many of the older FE aircraft don't even have folders that should be dropped into the Decals folder, only a local skin/texture folder that sits within the aircraft folder itself, so that can save a lot of time and hassle.
Happy flying,
Von S