There is only one Model-Editor. It is the 'General Assembly'.

Click the Model name-tab to make the Model-Editor active.

Model in Model-Editor

Model in Model-Editor

Default View : 

 Red, Green and Blue Arrows Triad:

These are +X, +Y and +Z-axes at the Origin of the machine-frame.

 Three[3] Orthogonal Planes.

The Planes are collinear with the  XY, XZ, and XZ axes

When to use:

Use the Model-Editor to:

Add Mechanism-Editors

Add Planes

Edit and view the Solids

Cycle to move mechanisms, each synchronized with the Master Machine Angle.

Model element : 

Use the Model Options dialog-box to edit the Model Lighting and to Synchronize CAD Files

Notes :  The Gravity Vector is always in the negative Y–axis direction [downwards]

Top-Tip : 

In the Model-Editor, it is in-convenient [annoying] that Solids turn red [default colour] as you move your mouse over them.

To stop this happening, add a filter for an element you cannot see in the Model-Editor. E.g.:

The Solids will not turn red as you move your mouse above Solid elements.

Tutorial and Reference Help Files for MechDesigner and MotionDesigner 13.2 + © Machine, Mechanism, Motion and Cam Design Software by PSMotion Ltd