Jim,
If your dynamo has been sat around for some time since having it overhauled, it may have lost it's residual magnetism. To overcome this - assuming the bike is otherwise ready to run - remove the 2 wires from the back of the dynamo. You will see that the 2 terminals are labelled 'D' and 'F'. With a spare piece of wire, connect the 'F' terminal to the live side of the battery for a couple of seconds (be careful, the wire may get hot!). This will restore the magnetism in the field coils. Having done this, replace the 2 original wires and start the engine - the dynamo should now provide a charge with the correct polarity for your battery.
Regards,
Eddie.