i had the same on the website but using the app seemed to work for any hotel in usa or france or uk...
10% off or one free night in ten. Same deal. Cold.
If its anything like pretty much every hotels.com code in the last 2 years non participating hotels means any that are from the major chains.
Please read the terms and conditions, there are some non-participating hotels. Best wishes. ;(
I tried few hotels for a Saturday night in Manchester every hotel said code invalid