You can always take a look at Microsofts CatalogManager, which they have as part of their CommerceServer product. And then do nothing like it, because, even though it is infinately better than their earlier versions, it is still not good.*
There are two problems that you have - firstly, you need to provide edits for categories and products, both in the same interface. This makes it tricky, becasue the interface needs to handle both of these. If you can utilise the mouse clicks on the treeview to handle adding new categories, that is probably a good idea.
The second problem is that you may need to work with quite a few products at once. So if you can make htis possible, that would be good. At the very least, make sure that adding 10% to all prices in a category, for example, is easy to do.
SO I would make the product and category addition from the treeview, and use as much space as possible to enable the editing of the product details. And make sure that the interactions are all very clear. Always remember - I am sure that you will - that the people using this will be product specialists, not IT specialists. Make sure that it matches what they want to do and how they want to do it.
- Their earlier versions are what drove me into HCI work, because they are wonderful examples of unusable user interfaces.