your 'copy ' of maya will let you unparent.., try re-installing.., if not, its not maya its 99% probability its you.., if your not sure down load the PLE version and see if that works.., or read the 'unparenting' in 'help' if you havent already.
try scaling in another direction.., you might have the projects rotations different from the tuts..., eg.., try scaling in Z... also remember when mirroring like that, by scaling in a direction.., the pivot point needs to be in the middle of the completed object.., which means it must be exactly on the edge of your model (before mirroring) when you scale..,
take it easy and life will be easy