Here is a simple tutorial will help you enable the Now Playing Pop-up in your dock. There is a hidden settings in your dock by which you can Enable or disable this notification.
How to Enable:
To enable this pop-up follow the following steps:
The very first step is to quit the iTunes application then go to applications and then Utilities; Launch the Terminal application from there.
Enter the following command in Terminal (you can copy paste it):
defaults write com.apple.dock itunes-notifications -bool TRUE;killall Dock
Start the iTunes and play any track, you will notice a pop-up in your Dock, showing which track is playing. But iTunes icon might be disappeared, however to get back this icon, Enter the following command;
defaults write com.apple.dock notification-always-show-image -bool TRUE;killall Dock
How to Disable:
To disable this pop up, open terminal window and Enter the following commands;
defaults delete com.apple.dock itunes-notificationsdefaults delete com.apple.dock notification-always-show-image;killall Dock
In this way, you can enable Now-Playing Pop-up on your Dock on Mac OS!