How can I operate "Find & Replace" using only the keyboard in Pages for OS X? When I have clicked Tab to highlight the Replace All button, Enter doesn't do anything. Do I absolutely have to use the mouse for this? 

It's inefficient to move between the keyboard and mouse when carrying out a long sequence of "Find &amp; Replace" actions.

This is Pages 5.0 (1465) running on Yosemite.
Use the Spacebar Space to click the selected button (not the Enter key Enter). That should do it for you.

(Note to editors: I want to have the word "Spacebar" in there in case someone searches for it.)