In the new online shop, to add a stock item to a category, follow this procedure:
(screens may vary depending on your preferred User Interface)
1) Launch EVE and click Data
2) Go to Stores and click Find
3) Click Search
4) Double click on your online store record
5) Click EVE OnLine tab
6) Click Menus + Categories
7) Click the Menu with the category where you wish to add item(s)
8) Click the category where you wish to add item(s)
9) Click the Tools icon
10) Click Add items…
11) The Stock Item search will open
12) Enter search criteria as you see fit
13) Click Search
14) Select the item(s) you wish to add. Hold CTRL pressed to select multiple items
15) Click Add. Your item(s) will be added to the corresponding category
16) Save the newly applied settings