You can hover your mouse over the little coloured squares on the right side of the screen and some of them kinda explain how they work. Then you can drag the dice from the left side of the screen onto them. You have to be fairly precise, but then the die should stick to the buff slot. If you weren't close enough, it won't get sucked in to the center of the slot. The buff slots should've been bigger so you don't have to be so precise so sorry about that.
You have a nice day too :D