Hey Comixian,
To get sockets in your Flipbook, you need to add a socket to each Sprite asset that Flipbook uses. Open each Sprite asset and look in Details panel. Click + button beside Sockets to add a Socket element. You can then rename and reposition Socket as desired.
You want to make sure you name each Socket same for every Sprite: If you want a socket to represent position of Left Hand, for example, create a Socket element and name it LeftHandSocket in each Sprite, and move it where you want it to be for that frame of Flipbook.
Hope that helps!