Setting up the opening balance in inventory does not add inventory only the value. Now that you are out of history mode the only way to add inventory would be through vendor invoices or through Inventory Adjustments. Using Inventory Adjustments you would need to know the value for each inventory Item so you can deduct it from the current value with zero quantity and add it back in again with the proper quantity.

Your inventory balance is unchanged since the beginning of the year so I presume you are intending to use the periodic inventory method.  However, the inventory module has been activated as if you had intended to use the perpetual inventory method.

To "turn off" the inventory module, go to Setup/User Preferences/View.  Uncheck Inventory and Services.  This will put Sage 50 into periodic mode and it will no longer be out of balance.