This is how I would design the layout editor.
Note the following points.
- No drag & drop. The user will not be overwhelmed by the infinite number of possible combinations.
- No mention of left/right sidebar/main panel. And elements are listed not by their location but by their functionality. Just select elements and their location (right/left/vertical, etc.) and MB will take care of the rest. Some elements will be shown on the left sidebar, and others on the left main panel, but the user doesn't need to care about that. The user will just take it as how things are designed with MB.
- Currently selected element (thumbnail browser on the mockup) will be highlighted on the right diagram. The shape of diagram will keep changing according to the selected elements and their selected locations so that the user could preview how it will look and quickly learn the name of each element.