I have to search for this every time I set up a new mac…
I don’t like to use the “mouse” (trackpad in my case) when I don’t have to, but any mac in a default setup makes full keyboard navigation impossible, particularly where dialog boxes are concerned—unless you enable full keyboard access.
System Preferences > Keyboard > Keyboard Shortcuts and select the radio button “All Controls” under the option “Full Keyboard Access: In windows and dialogs, press Tab to move keyboard focus between:”
This will allow you to get around just about everywhere just by tabbing and arrowing. When presented with a dialog box with two or more buttons, you will note that aside from the default button (solid blue, activates with return), there is another highlighted button which is activated when the space bar is pressed. You can tab between each button to highlight it, and in each case, the highlighted button will activate with the space bar.