After updated to OSX mavericks 10.9.2, I can't input any keys and so can't start the game.May someone help me?Thank you.
It seems unlikely that upgrading your OS was responsible for this (but anything is possible...).

Two thoughts:
1. What CoinMode are you in? If you are in "pay" you won't be able to start the game until credits are entered. You can toggle between CoinModes by holding [fn] and [f3] and tapping [1]
2. Have you tried reinstalling StepMania 5? You can download the newest version, Beta 3, here: https://github.com/stepmania/stepmania/releases/tag/SM5-beta3