Add-ons are additional items that can be added to pledge level. You will not be able to select what those add-ons now, but you do need to pledge the extra dollars now.
If you want an add-on item, just adjust your pledge amount to be the pledge level plus the total cost of add-on items. You still need to select one of the pledge levels.
Here's an example: You want 3 copies of the game, 3 metal coins and 1 t-shirt.
You should pledge for the "Cavalry" level pledge (2 games + 2 coins for $90) and then add-on 1 copy of the game ($35), 1 coin ($10) and 1 t-shirt ($20). You will select the "Cavalry" level reward when you pledge, but the dollar amount should be adjusted to $155.