I can't see the category I am looking for.

On the Data Sheet, if you select a Department from the menu in the top left, and a Category from the menu at the top left, and you see no way to edit the data, as below:

the reason is because that Category is not assigned to the Department, so it is not possible to enter data.

To rectify this, go to Settings > Manage Categories or Manage Departments, and add the Category to the Department.