If you need to integrate your online store, you can look at this help center article here.
How to Add SKUs to Dashboard Manually
1. Add SKUs to your Dashboard by first going to the "Inventory" tab on the left hand side.
2. Once In your "Inventory" tab, Click on top option "Add/View Products".
3. Once in "Add/View Products", click on the top link "Add New Product"
4. Once you click on the link, you will be prompted to "Add Product Name" to fill out accordingly. Then click the "Save" button.
How to Add SKUs From Your Integrated Store(s)
1. Click Inventory-->Add/View Products, then "Sync from Marketplace" which will Sync in the Dashboard.