As @Graham42 said, from version 2.1 mouse options has been renamed but you can use the mouse with any version of tmux adding this to your ~/.tmux.conf:
Bash shells:
is_pre_2_1="[[ $(tmux -V | cut -d' ' -f2) < 2.1 ]] && echo true || echo false"
if-shell "$is_pre_2_1" "setw -g mode-mouse on; set -g mouse-resize-pane on;\
set -g mouse-select-pane on; set -g mouse-select-window on" "set -g mouse on"
Sh (Bourne shell) shells:
is_pre_2_1="tmux -V | cut -d' ' -f2 | awk '{print ($0 < 2.1) ? "true" : "false"}'"
if-shell "$is_pre_2_1" "setw -g mode-mouse on; set -g mouse-resize-pane on;\
set -g mouse-select-pane on; set -g mouse-select-window on" "set -g mouse on"
Hope this helps