The term 'meta key' is used mostly in Emacs and bash. It's the "other" control key, giving you additional commands beyond the real Control key. The Control key is usually labelled 'control' or 'Ctrl' on keyboards, and doesn't vary much (though see note below). But Meta is no longer on modern keyboards, so the key you actually press may vary.
On the Mac, Meta may not be mapped to anything. You might have to press Esc, then x to generate M-x. Various applications decide how the meta key is mapped. For example, in Terminal.app, the Keyboard preference pane has a checkbox to let you use option as the meta key. In Aquamacs (the Mac windowed version of Emacs), meta defaults to the option key, but can be changed to the command key.
This is supposedly different on different systems, but I think it's been the control key on all the systems I recall using. You can find out by opening Emacs and trying to get the cursor to the mini-buffer at the bottom.
Try in order
Ctrl-x,
Alt-x (Option-x on the Mac)
Esc + x
the right one is the one that moves the cursor to the line at the bottom where it's prompting you to type something else in.