In the picture below, click on the symbol marked no.1. When you do this the mooring points for the barge will light up in red colour (no 2) Click on each red ball to let go of the mooring to the barge. As you click the red balls will turn yellow indicating that that particular line is released. To fasten to a barge click on the yellow balls when the corresponding balls will be illuminated on the barge and you can then connect.
Angus.