If there's just one option hitting <kbd>Space</kbd> or <kbd>Enter</kbd> should do the trick, as the button should be preselected. If there are more than one option you could tab between them by enabling  **All Controls** under **System Preferences / Keyboard / Full Keyboard Access**. 

Then tabbing through the options, a subtle blue glow will highlight the *tabbed* option. Hitting <kbd>Space</kbd> will select it, while hitting <kbd>Enter</kbd> will select the default option.