Navigating OS X dialogs with the keyboard
OS X 10.10 "Yosemite" (and older versions at least back to OS X 10.7 "Lion") lets you use the Tab key to navigate between buttons (and other items) on the screen. To configure it select "All controls" in System Preferences>Keyboard>Shortcuts as shown below:



When the desired button has focus, press the Space bar to select it. (Don't press Enter, as it will select the default button.)