Based on the different things people are experiencing to get it working, I guess people will have to just try each and hope one works.
I just got mine working (iPhone 5). My iPhone 4 worked fine. Haven't checked my ipads yet.
Anyway, just thought I'd list some things others gave, but there was sometimes something missing. In my case, I delete music still on the iphone even though iTunes Match was on. I did them individually (before I saw more detailed instructions here). Fortunately, I only had 44 songs.
Anyway, the path you in Settings is:
Settings>>>General>>>Usage>>>Music
One set of instructions left out "General" which caused me a lot of confusion, especially since there is a "Music" item directly in Settings without going to "General".
Next, it was pointed out that in "Usage", you then select "Music" after the list populates. However, if you are like me, at the end of list of apps, you will have to tap "Show all Apps" because "Music" didn't show in my initial list.
In my case, I had to scroll through a LOT of apps to find "Music". I don't know if it will be the same for everyone, but it turned out to be the last one on my phone so it might be worthwhile to quickly scroll to the end to see. If not there, you can just scroll more slowly back up.
I assume everything else is how others described - I can't check because I had already deleted the music on my iphone so next to "Music" it says "No Data" so tapping it doesn't do anything right now for me.
Hope this helps in case someone went to the wrong "Music" item or something.