Yo Trucker: I didn't have any trouble that I can recall with the Maule (doesn't that package self-install?), but I think
maybe your trouble is that what you thought was the folder that FS9 looks for to set up the roster of available planes is really the folder
containing the folder you want in your "Aircraft" folder. FS9 searches by folder name, so if it sees the wrong one there, it will delve no further, and simply not show it.
Been a while since I installed my Maule, but looking at my "Aircraft" folder, I see that the title of the folder is: wait, there are a bunch of folders! Bet you didn't separate them:
If you open FS9 as a program folder (Programs>Microsoft Games> Flight Simulator 9), in the "Aircraft" subfolder you should see:
Maule M7-260 Amphibian
Maule M7-260 Documents
Maule M7-260 Panels
Maule M7-260 Sounds
Maule M7-260 Spring Gear
Maule M7-260 Taildragger
Maule M7-260 Tri-cycle
Maule M7-260 Tundra Wheel
Maule M7-260 Wheel-ski
Not sure if I installed it "right"myself, but mine works. and she's a real nice one...enjoy!!
The Lyons' Tri-Pacer was mentioned too: if you liked their Aztec, you're gonna
love the Tri-Pacer.
As for the round-trip flight plan (annoying, isn't it?), Gary's suggestion is your best bet. It will also come in handy when ATC just cancels your plan out of the bue (WTF?!), or if you need to divert. Selecting "CReate or file IFR plan" in the ATC window, the sim stops and takes you to the flight planning page. You can also create any plans you might need beforehand, save them, then load them from the Flight Plan page when the time comes, instead of taking the time then (mid-flight)to do all that clicking and dragging, etc.
This is one of the reasons sim flying is harder than the real thing...

Good luck!