The whiteboard library is a Premium feature.
You can load a copy of shared whiteboards directly to your library.
To share a whiteboard, navigate to your Teacher Dashboard and click on Library to show your whiteboard library. Then, scroll down and click on Load shared library. This brings up a popup. Enter the shared whiteboard code given to you, and click on Load shared library.
When loading a shared whiteboard, you load a copy which is made at the time of loading. This means that changes you make to the whiteboard are not shared to the original whiteboard, or vice versa.