I\'ve been tweaking my ~/.vim quite a lot lately, and it seems that I broke something.
I\'m using the CommandT plugin, which when the search is open all
You can list all current mappings by typing :map inside the command window.
You could also clear all maps using :mapclear and then map <C-S> again.
It could be that you vimrc contains alternate maps depending on the environment. Look for things like has("gui_running").
Finally when running MacVim gvimrc is sourced vs. vimrc when running vim in the terminal.
Depending on the differences in both of those, mappings and other features will change.
I noticed, that you are talking about Cmd key in the title, but about Ctrl in your description. The Cmd key does not work in terminal vim.
If you are talking about the Cmd key, consult my answer to a similar question.
I've found solution for this problem here: https://apple.stackexchange.com/a/34503
Just put in your .bashrc / .zshrc this line of code:
stty -ixon -ixoff