Cannot assign categories to MarketPress product

I'm using MarketPress to sell house plans. I use User Role Manager to create a custom role "Architect". I understand that I have to enable certain capabilities to allow Architects to create/manage products. Everything seems to work fine except that they can't assign a category to the products they create.

Here's a screenshot of the "Architects" role capabilities:
http://fevaag.no/role.jpg

Here's a screenshot of the product entry page. Notice that the list of categories is there but neither can be selected.
http://fevaag.no/product-entry.jpg

I understand that to allow them to manage products, I had to enable "edit_pages", "delete pages", "edit_published_pages", etc.

My question is: What capability do I have to check to allow architects to assign categories to a product?