When one gets a Mac, the first thing to do is open a terminal.
$ apt-get install tmux irssi
bash: apt-get: command not found.
$ pacman -S tmux irssi
bash: pacman: command not found.
$ yum install tmux irssi
bash: yum: command not found.
The second thing to do is remember that OS X doesn't have a package manager, for some insane reason. The third thing is to go install Homebrew.
$ ruby -e "$(curl -fsSLk https://gist.github.com/raw/323731/install_homebrew.rb)"
And then you can do the stuff you were trying to do in the first place.
$ brew install tmux irssi
Now you have a working package manager, you should also:
$ brew install mutt cmus gvim
And now you have all the software necessary.