i did a lot of research on this before my purchase, but the sales person threw them in for free, so i didn't get a chance to put them on myself... but i remember there should be three screws, two shorter ones on the side and one LONG screw in the middle, and it is this long one that keeps giving people trouble while installing, because it is kind of blocked (partially) by the rear wheel (hard to access)...
it sounds like that you will either need a special screw driver, or you may opt to take the rear wheel off... pre-drilling the rear mud guards might also be required...
a lot of discussions have been posted in this category, you might want to do some searching... good luck....
Put mine on a few weeks ago. Very easy install. Three screws, two existing screw holes and one you have to make. By jacking the X up it allowed the wheel to drop so I was able to get my drill in to make a pilot hole through the plastic for the third screw. If you have a very short (knubby) phillips screw driver or a small ratchet with a phillips bit you will not have to pull the wheels to screw that screw.
You will need a very short stubby screwdriver or compromise like I did. I used a driver from a power screwdriver set clamped into a pair of vice-grips. The space between the wheel well and the tire is very tight. Of course you could always remove the wheels
Learn from me on what NOT to do when installing the rear mud flaps:
1) the existing screws on MDX are not long enough to be used on mud flaps. Go to hardware store and buy long black screws.
2) don't use silver/chrome screws as substitutes...they look gawdy...trust me, I didn't have any black ones in my garage
3) jack I bought at Wal-mart does not lift the MDX high enough to get tires/wheels off, dang it....had to use the Acura supplied jack...if you buy a jack, make sure it states that it's eligible for SUV use
When you take wheel off, take time to view and appreciate the suspension system.