You do indeed have the correct syntax for changing the command prefix in tmux. But unfortunately you cannot use the combination Ctrl+Alt as a command prefix. Both Ctrl and Alt are only used in combination with other keys, but never alone and never with each other. For example,
C-b
stands for Ctrl+B
M-n
stands for Alt+N
Your attempt to bind M-C
actually means Alt+Shift+C (meta prefix with uppercase C). Similarly, C-M
would mean Ctrl+Shift+M.
In short, you have to combine Ctrl with some other key or Alt with some other key, not with each other.