If you are using iTerm2, you can copy text in Tmux session, holding down the Option key while dragging the mouse to make selection.
Then it should be possible to paste text anywhere with Cmd + V as usual.
Found it here: http://web.archive.org/web/20131226003700/http://ootput.wordpress.com/2013/08/02/copy-and-paste-in-tmux-with-mouse/