If any other addon you use has taken the standard Slew key events, there's just nothing we can do to prevent that so, you must disable that add-on while editing a parking.
On P3D V4, we also use the mouse, but it's not possible to do that in FSX, since we need a specific set of functions that are only available in P3D so, on FSX, you are stuck using keys, and you must ensure no other add-on is already using them.