tmux 2.0 on OSX: ~/.tmux.conf doesn't work
For some reason tmux didn't reload the ~/.tmux.conf when doing just exit and tmux again.
So I addressed this issue with sudo kill $(pidof tmux). After starting new tmux session all worked properly
You can reload your config with tmux command:
:source-file ~/.tmux.conf
To access tmux command line type Ctrl + b (or whatever your prefix is) followed by : .
More details could be found here.