Solids apply to MD-Solids - those Solids that are native to MechDesigner.
Use the Solids menu and Solids toolbar to add Profiles*, Holes, Polylines, and Auto-Profiles
To see Solids, you must use do Visibility toolbar > Show Solids in Mechanisms
See also CAD-Line dialog-box to import CAD-Solids from 3D-CAD.
The Solids toolbar is above the graphic-area