I was too worried about this. I was really not able to understand the intention why they have made touchpad or mouse use compulsory. For almost one and half year I have touched my touchpad whenever I got option buttons(dialogue buttons) i.e. cancel, ok, save, quit,restart, sleep etc. Then suddenly I realize that I am wrong all the time there is option to switch the keyboard buttons in action for these petty but all the time used things also. I don’t able to figure out why they have disabled this option by default that really is wrong.
So here is the way to enable keyboard tab button to switch between options in dialogue box.

Go to System Preferences -> Keyboard -> Keyboard Shortcuts and then check mark the option “All Controls” at the bottom.