As said in a pleasure, tmux 2.1 does not support separate mouse settings. He replaced them with one mouse setting, which can be set using set -g mouse on in your ~ / .tmux.conf file.
Regarding the installation of pane-base-index , a proposal by Mark Nichols:
# set window and pane index to 1 (0 by default) set-option -g base-index 1 setw -g pane-base-index 1
worked for me on OS X 10.10.5 running tmux 2.1, installed via brew in iTerm 2.1.4.
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