You pick arithmetic first on the drop down menu, then a popup menu displays 1 + 1, click on the first one and type how much damage you want or you could use a variable then click ok. It will then return to the previous 1 + 1 popup, the first value being changed to what you picked. Then click the right one and pick Convert Integer to Real, another popup will show, there you pick Hero Attribute.
EDIT: I attached a map, it has a trigger with the action you need.