My interpretation is that the green terminal may be a chassis connection.
For your purposes, I would ignore the green terminal, and instead just use the red terminal for + and the black terminal for -. You could optionally connect the black terminal to the green terminal.
That interpretation is based on my reading of the schematic at http://www.multimeterwarehouse.com/P...ply/HY3020.pdf (which may be for the identical power supply).
You should be able to verify that suggestion by using a multimeter to read the voltage between the red and black terminals.