When you buy a scroll, you then have to go to your inventory and drag and drop it on the equipment of your choice.
If the equipment already had a scroll, the effect if replaced by the new one. they don't stack.
but if you have, say, 3 scrolls of virility (one on eaqh equipment piece) they cumulate and so you have +6 in virility.
You can always check on your equipment stats by clicking on it in the inventory.
We will replace the current "tuto" in the next update so it better explains how it works.