Navigate on pop up windows When I was using Windows I could navigate with tab to pop up windows for choosing 'Yes' or 'No' faster. Now on OS X  I haven't found a way. In some cases I can choose the first letter and it will be accepted. But in examples like in the picture below, there's no such a thing.
Go to the System preferences > Keyboard > Shortcuts.

Then you can enable Full Keyboard Access to All Controls !