Add a product to a category

From the Product

Click on your product in the list

Click the Category tab against that product

Click the Edit button

Check a category (you can also check more than one) for that product and click OK

Click Save in the top right

From the Category

Click Product in the left menu and select the Category tab

Click on your chosen category in the list

Click on the Products tab and click Add

Check your desired products and click OK

Click Save when you are done