Updated the image for the material context button
Fixed inconsistencies of terminology.

The "3D View Editor" was referred to as the "3D View Window" in several
places.
Added missing commands in "Surf. Class" section.

There were a couple sections that didn't tell the user to switch back
into Object Mode. The Plane.001 object was not being added to the Model
Objects list.

Also, some potentially ambiguous sections were clarified.
Fixed file and directory name typos.

sc_rxn was referred to as sc_rxns.

Scene.reactions.mdl was referrred to as sc_rxns.reactions.mdl

The reference to the installation instructions was fixed.