Tmux scrolling is a little different than just scrolling with a mouse wheel or a trackpad.
In tmux, scrolling is accomplished by first sending 'prefix-['. You'll see a little yellow box in the upper, right-hand corner of your terminal window.
In this mode, you can use arrow keys, PageUp, PageDown, Ctrl-U, Ctrl-D, hjkl (vi-mode), or other configurable movement keys to scroll forward or back through the buffer.
There is an option in relatively recent builds of tmux that allows you to use the mouse while in scrolling mode.
add this to your .tmux.conf
set-option -g mouse on Additionally, if memory serves, tmux is 'built into' iTerm, and thus features like multiplexing are built in as well. Is there a reason you want to use tmux in iTerm? The built-in OSX Terminal is definitely enhanced by a multiplexer like tmux, but iTerm seeks to alleviate those shortcomings by being a standalone solution.
EDIT 1: After upgrading to El Capitan, I noticed some unfamiliar behavior when using Terminal.app and tmux. This led me to an excellent article detailing options I wasn't aware of and which look to answer your question more thoroughly than I could hope to. All the options are tmux-specific, as far as I can tell, and are independent of OSX versions, so using them in Yosemite should be fine. Your scrolling question is covered here, with options like:
bind-key -t vi-copy WheelUpPane scroll-up bind-key -t vi-copy WheelDownPane scroll-down Many more useful options in the article linked below.
https://ryanfb.github.io/etc/2015/10/19/tmux_mouse_mode_on_el_capitan.html