I can’t find the motion library key

If you are looking to add the motion library behaviors to your puppet, you do so in rigging. Select the root of the puppet layers and click on the behaviors column to open the behavior menu. You will need to add the body, limb IK and motion library behaviors. Many of the example puppets are already rigged for the motion library. Once your puppet is rigged, you can access the motion library from the properties panel to the right of the stage. Here's more information on using the motion library. Motion Library