mouse wheel isn't scrolling in tmux

Correct command is:

set -g mode-mouse on

You can also add it to your ~/.tmux.conf

Just to link two relevant answers together:

Why does tmux set -g mouse-mode on no longer work?

As pointed out in this answer, the option got renamed since tmux version 2.1 to:

tmux set -g mouse on

To find your local tmux version, just run tmux -V.