If you have 30 Polished Metal in your Inventory, then the first part of this quest will already be completed. If not, then send some Polished Metal to your neighbours as Gifts, and they should return the favour.
When you click on the Carnival Pavilion to complete the second part of the quest, the 30 Polished Metal will be removed from your inventory. It will then take 12 hours to hammer these into place on the construction. You can Speed Up the hammering with 2,088 Food.