One of the Readme.txt has the steps
If you want to use the trainer, follow these steps to launch the game!
For Window Store or XBOX Pass, launch the game using the included shortcut (says "Launch Xbox Pass Halo Reach CH".
Once the game starts, you can then attach the trainer to it.
Yes that's the first thing I did, launched with the provided shortcut. Don't know why I said launcher, I meant shortcut. When that didn't work as trainer couldn't find game ( yes waited until I picked the game and loaded into before trying to activate trainer). That's when I tried other methods of launching and seeing if the trainer could find the game.
Very weird one as previous versions of the trainer worked.