You should have gotten a piece of paper in house on which a spell is written its looks like a normal piece of paper if i remember correctly then you need to go down to the place in which you got the staff of pontar (don't remember the correct name) and just after that you will enter small closed off area on your party member will tell you that something looks odd here ( or something like that ) and just run around this place and use the scroll with the spell like a spell ( pest to add it to a quick bar ) and it somewhere there
I am bad in explaining things but hope it helps
