Again, I urge you to *not* do anything until you can properly measure that voltage! You'll kick yourself in the head (not an easy task) if you fry your Jakk's game first time out!
When I was looking for transformer for my unit, I measured some supposedly 6v units that were putting out ten volts!
Once you have a multimeter, you can find easily which is negative. If you read, and it displays a positive number, your black lead is on the negative. If it reads negative, the other wire is.
I don't know how the Radio Shacks around you are, but you could take the unit there and have it tested. I'm actually blessed to have two decent units nearby