Tow in D3 -- no overdrive. Keep the MDX out of drive the whole time -- don't waste your time with trying to use the OD when on flat roads. Set it and forget it.
I know, here comes my first panda -- we have towed off and on with our Honda Odyssey for years. Same engine tranny set up.
I went to the dealer for the tow package. The guys there said I could't tow my boat with it.
despite the manual clearly saying I can and their selling a way overpriced tow package. I called north American Honda and they put me on the phone with an engineer. His explanation was keep it out of Drive, and that these gas engines and tranny's have the most torque when the RPMs are high, so this is where they are most efficient.
I did my own tow package -- screw the dealership.
etrailer has a video on cleaning out and re-threading for installing hitches.
Honda transmissions are supposedly crap. I sold our first Odyssey with the original tranny at 160K and towed significantly with hit. You will be fine once you get that hitch on there.
PS: I have a nice truck for towing, and may daughters jeep tows, but I will still hitch up the Nautique to the Honda when the trip calls for the room of a van.