When you have more than 1 location, you will need to manage inventory for all of them. The important part of managing inventory is your current location context. When logging in you will be asked to select a location (if employee has access to more than 1). You can also change the context in the top of the navigation header on the right. When viewing the items screen it will show inventory for that location. When editing an item; you will be able to set inventory for all locations you have access to.
There are a couple ways to add/update inventory. The easiest way is to make sure you have the right location context.
Receivings module: You scan scan/type items to receive. Then it will add to your current locations inventory
Manual edit via items form: You can edit an item and set inventory for all available locations
Excel Import: you can do an excel import and set the quantity. NOTE that when doing an excel import it will only update the current location that is selected