If you are getting shocks to earth or measuring voltage to earth from the casing of the inverter, something doesn't add up here. It says in the instructions linked by James that the earth pin of the output socket is connected to the case, so if the plug is connected to the DB and the DB is earthed, then the case should be too. The instructions may be incorrect as these cheap units are prone to rapid design changes; do a continuity check from case to DB earth of the inverter's socket, and then onwards to the earth bar in the DB.
On a similar note, is the neutral earthed within the inverter? If not, have you earthed in in the fuseboard? Not doing this won't give you shocks from the case though.