The symbol definitions can be found in the Notation section of the user manual.

When importing data, Section Maker will connect together any lines whose ends are within 5 mm (0.1969 inches) of each other. If your lines touch in your CAD system then the rescaling should not affect this.
 
Once you have placed the DXF shape into Section Maker the properties will be displayed in the Properties window. You will need to assign materials to the shape to compute these properties correctly.

All the commands in the Groups submenu are only available when you have the Groups tab in the Library window activated. Select the Library window from the Windows menu and select the Groups tab.