If you've learned a spell/ability, you then have to 'Equip' it in the abilities menu. This basically assigns it as an option for your ability quickslot toggling ('up d-pad'). Once you have the desired spell/ability on the quickslot, hold R2/Right Trigger. It takes some time to cast certain ones such as healing.
Most magic is used in this way, though others take an effect automatically once equipped.
