# Short answer:
# to use the mouse, add this to your ~/.tmux.conf file
set -g mouse on

# Long answer (shamelessly recycled from Dennis Williamson):
# You can use your normal navigation keys (e.g. Up Arrow, PgDn, etc) to scroll
# in copy mode:
# 	Type Ctrl-b then [ to enter copy mode. Press q to quit copy mode

# Alternatively you can press Ctrl-b PgUp to go directly into copy mode and 
# scroll one page up

# If you've added "set-window-option -g mode-keys vi" to your .tmux.conf file,
# once in copy mode you can also scroll the page up/down line by line using
# Shift-k and Shift-j. Unshifted, the cursor moves instead of the page.

# Other vi/emacs navigation commands from the man page:

Function                     vi              emacs
--------                     --              -----
Top of history               g               M->
Bottom of history            G               M-<
Half page down               C-d             M-Down
Half page up                 C-u             M-Up
Next page                    C-f             Page down
Previous page                C-b             Page up
Scroll down                  C-Down or C-e   C-Down
Scroll up                    C-Up or C-y     C-Up
Go to line                   :               g
Search backward              ?               C-r
Search forward               /               C-s
Search again                 n               n
Search again in reverse      N               N