The Mouse System Preferences pane does not include an option to enable momentum scrolling. The setting is a hidden preference. Use following command in Terminal to enable momentum scrolling.
defaults write com.apple.driver.AppleBluetoothMultitouch.mouse MouseMomentumScroll -bool yes
Revert to the default setting:
defaults write com.apple.driver.AppleBluetoothMultitouch.mouse MouseMomentumScroll -bool no
Screencast: