I think what your dad means is do not glue the weights to the shell (good advice). Anywhere on the (probably metal) frame is fine. As long as the weight is not touching the motor or any bare (unlikely) wires is ok. It's ok f it touches the metal frame. If you do place it in the wrong location (electrically speaking) you will get an instant short circuit which will be equally instantly obvious, lol.
If I was doing this I would take the shell off, place the loco on the track , carefully place weights on it (one at a time in different locations), then very slowly and carefully give it some power. If you have a short you will immediately know and can move the weight as necessary. Once you determine the best locations and how much to add - then carefully glue them in place. Don't over do the glue.
Just think ahead, go slow, and don't overload the motor. Sometimes finding a replacement can be difficult.
BTW - assuming you bought this new you should have gotten a data sheet with it which will (well its supposed to, lol) tell you the maximum amount of current the motor is designed to draw. DO NOT EXCEDE THIS VALUE!
Ben