If you don't need to track inventory in multiple locations, just turn the feature off. QuickBooks will hide any references to site in the interface, though your data remains intact. You can continue opening and using the file as you normally would.
If your company has subscribed to Advanced Inventory, you just have to enable your copy of QuickBooks to use this feature. Select this choice and QuickBooks will verify your subscription and you can begin using QuickBooks Enterprise Solutions Advanced Inventory.
If you want to subscribe to Advanced Inventory and track inventory at multiple locations, select this option.