at the start it says to cast Metamorphosis [mod], meaning if no mod key is pressed it will execute this command.
No, it means that if you are holding a modifier key while activating the macro, it will cast Metamorphosis.

http://www.macroexplain.com/

I tried changing the macro to [Mod;Ctrl] or tab or whatever and nothing I do makes Metamorphosis fire off.
The format is mod:ctrl not mod;ctrl. You also need to be sure that you don't already have Ctrl+whatever key already bound to something else. Or in the case of using a different modifier, that mod+key combination isn't already bound to something. Tab is not a valid modifier.