For example: You trained the first point into "Sins of Flesh" - you will get 2 things: One is the abillity which must be placed into the AA Slots (You have 6 of them 2 blue, 2 red and 2 silver) the second thing you get is the Combo itself, you just place it in your shortcut bar (where you have all your other spells / combos and so on too) and if you click on it you start the new combo just like all other.
Its a bit confusing but so to say you got 2 times the same icon (abillity activator for AA slots and the combo icon) They are both found on different places: The one you need to put into the AA slot is found on the right side of you combos and spells window, the second one that you need to start the combo is found on the right side under AA combos.
EDIT: I made a picture, maybe it's a bit easier to understand like this ^^
http://www.bilder-upload.eu/show.php...1439732389.png