To enable the iTunes "Now Playing" notification in the dock, open a terminal and type the following commands:

defaults write com.apple.dock itunes-notifications -bool TRUE

killall Dock

Now the notification feature is enabled, you simply need to add the iTunes icon to your dock. Open a terminal and type the following commands:

defaults write com.apple.dock itunes-notifications -bool TRUE

killall Dock

Note that: To disable this feature, use the following commands