How can I activate buttons with just the keyboard?
Enable Keyboard Control of the UI

System Prefs &gt; Keyboard &gt; Shortcuts


Then select "All Controls" radio button at the bottom, rather than just "Text boxes and lists only".



Keyboard Shortcuts


&#x21E5; (TAB) will move between buttons.
esc (ESC) is cancel.
space (SPACE) selects the active button (blue, outline).
&#x21A9; (RETURN) is OK or the default button (blue, pulsing, filled).
For some dialog boxes, ⌘+first_letter will select the button with a certain first letter in the text (as pointed out by @Griffo).