@zarfire
The riding scrolls you purchase should be clickable. Each click on a stack of scrolls will grant a single lesson of the type (stamina lesson for stamina scrolls, speed lesson for speed scrolls, etc.) There is no real interface for it - you just use the scroll by clicking it and the boost applies to that character's riding skill. I have never purchased riding lessons from the crown store, but have won some in the *cough* crates. Just banked them on my maxed out character and then retrieved them on my lowbie alts and boosted the skill up one click at a time.

Hope that helps!