What size is that fuse? Is it a small 10 or 15 amp fuse for the field winding or a large bolted-in fuse for the generator's output wire? When does it blow, right away or after the engine has been started?
If it blows as soon as the battery is connected, connect a test light in place of the fuse. If it's bright, wiggle and move wires and disconnect different things to see what makes the short go away. When it does, the light will go off.