Praising is tied to the interaction system, it used to be part of the "talk" option long ago if memory serves. There is a button just below Quest Log called Interaction in the main mansion screen. From there, just select the slave you wish to interact with, the options you are specificaly looking for are under "Discipline".
Praising can be used twice to significantly lower your slaves stress and give a small boost to loyalty, but it doesnt work if your slave is not near or at max obedience. "Scold" increases fear and stress, which can be usefull if you are low on mana and dont have a fear spell handy. You can find a lot of other stuff that either increases obedience, loyalty or fear, and the "Teach" option under "Actions" increases learning points. You generaly dont want to increase stress, but most actions that increase fear, increase stress and, teaching, the best way to gain learning points gives small amounts of stress.