The below steps are detailed for the V7.0 version. If you are using V6.0, the steps will be slightly different.
Double click “B” area and then right click it directly, check the “plan” item and click [OK] button in the “Set Piece” dialog box, then you can get the gusset piece. > Double click “C” area and then right click it directly, check the “plan” item and click [OK] button in the “Set Piece” dialog box, then you can get back piece.
2)How to add seam allowance: http://www.padsystem.com/en/assets/documents/en/pattern/pattern_design_en_ch3.html#tool-image163-seam-allowance