To use any ability go to the Magic screen, in there you have Powers - all your powers and abilities will be listed there. Add any of them as Favorite then you can assign them from the quick menu while playing - same way as you equip weapon. When assigned, click RB button.
More controls can be found here.
As for scrolls, same way - they are accessible from within the Inventory, you have sub menu for them. You can either assign them directly from there, or have them as favorite then assign from the quick menu. Once assigned click RB to activate.