Dynamo light without rear light connected?
On old bikes usually the (steel) frame was used as second wire for closing the circuit. The old dynamo have one wire going out and some sort of built in electrical continuity with the frame. 

First of all check that the bulb works, by using a trusted electricity source. Then:


If you bought a modern dynamo and a modern light, just lay an
additional wire between the two.
If you instead want to use a new dynamo with the old light, make sure
you connect one of the two outputs from the dynamo to the frame (a
washer in the right place will do the trick)




Not requested by your question: The rear light usually had connection to the frame via the mud shield (either it was made of metal, or had a metallic strip in the plastic body.