On centos 6, the tmux version is too old to be used with iTerm integration available on OSX. Either, you run centos7 and use yum, or you download a new version of Tmux from the official web site and compile it:
On the server, create a new detached session named nickyvm-tmux for example:
nickyvm$: tmux new -s nickyvm-tmux -d
Afterwards, on osx make an alias that matches the configution below :
alias stmux_vm="ssh nickyvm -t 'tmux -CC attach -t nickyvm-tmux'"
Source again your profile, open iTerm and call the alias. Enjoy