OK yay! Even though I was careful to put (drag) all the invisible files and htaccess inside my grav folder within MAMP htdocs, I decided to trash that grav from my MAMP, went back and got my Grav.zip, put THAT in the htdocs folder (like the directions said in the first place, sorry) and reopened it inside my htdocs directory and now it works fine.
No more 404 when clicking the menu link to the new page. Onward.