Are you using the 5E by Roll20 sheet? If so you don't want to add it in your attacks, as that would be overwritten if you so much as look at the spell on the spells page of the sheet. Go to the spells page and open the spell to edit. Add 2+the spell's base level to the healing amount, so for Cure Wounds you'd add +3 to the line with the 1d8. In the Higher Level spell section at the bottom where it says add 1 d8 there's a place to add a flat modifier on the end, so add 1 there.
