OnEffectStart() is the correct approach if you want the script to something when the scroll's spell is used. Note though: The script must be attached to the MagicEffect not the Spell/Scroll.
Something like that should be working:
Scriptname EffectTest extends ActiveMagicEffectEvent OnEffectStart(Actor akTarget, Actor akCaster) Debug.MessageBox("It works!!!")EndEventAttach that Script to the Magic Effect of your Scroll and it should be working.