How and where do I add the value of inventory at the beginning of the calendar year?

This only applies to folks using the accrual inventory method

The value of inventory at the beginning of the year will be calculated from the purchase price and purchase date on current inventory and sales. So if you have purchase price/purchase date assigned to each inventory item and to each sale made this year, you will have your beginning inventory balance. You can see this number by running an "Inventory Report" (How do I view beginning inventory balance, ending inventory balance, and purchases during the year?)