I have the required setw / setg setting for mouse in my .tmux.conf but they aren't working.
I've done tmux source-file .tmux.conf
I can use tmux and split windows with my remapped keys and other settings that I have such as remapping the power key ARE working.
Best Answer
Turned out I was using tmux with the basic mac terminal app.
It mostly worked but a few things didn't. The mouse functionality is an example.
The answer was that I needed to download and use iterm2 as the actual terminal app to use tmux with.
Once I did this my tmux mouse settings worked fine with it.