Hi! It could be that you’re in object or edit mode since the pose library is only used in pose mode. For a full rundown on armatures and bones, I’d recommend taking our fundamentals of rigging course and the previous course about rigging the rifle:
https://cgcookie.com/course/fundamentals-of-rigging
https://cgcookie.com/course/rigging-a-transforming-rifle-in-blender-2-8
Happy animating!
