You'll have to peel back the protective film over where the battery wires exit the battery itself. There should be a couple of tabs that the original battery wires were soldered to. You'll want to carefully solder your wire onto the tab in place of the missing wire. Once done, get a multimeter and check for voltage. This does two things. The most obvious is to ensure the connection is a good one. The less obvious is so you know which lead is the positive, assuming there are no markings or other telltale indicators.
P.S. When you figure out which lead is positive, take a piece of tape - preferably electrical tape, although any kind will do - and fold it over the wire so it looks like a flag.