What I did was to make a folder called AFCAD and put it into the addonscenery folder. I then made a scenery folder inside of the afcad folder than as you download or make new AFCAD files just put them there . The nice thing about this way is you only have to activate it once. Any time you add new files it will always add them at the start of FS. 8)
Mind you this is just one way of doing it. But I like to make folders for each type of scenery I add that way I know wher to find it.