When you are in your actions/traits screen, or in any other screen where you would want to key-bind something, press Square to bring up a key-bind interface. From here, just press the button combination you want to use for that ability. If you are in a screen like the inventory where this doesn't quite apply, you can use the touch pad or a USB mouse to drag and drop the icon. With the touch pad, move the cursor over the item and tap the touch pad to "pick up" the icon, then drag it over to the desired slot and tap it again to place it.

Hope this helps!