Answer:
In the Steel Joints add-on, the order of components plays an important role when editing a joint: It "runs" from top to bottom.
For example, if you want to edit a plate, you have to define the "Plate" in the list of components first, before defining the "Plate Editor" component. The same principle applies to other components, such as "Plate Cut".
The attached video shows in detail how to proceed when modeling without using the component library.
