Can an employer MANDATE employees to work for the weekend (over & above reg wk) for training on new operating system?

Your question has two parts. The first is whether an ER can require you to work on the weekend. Generally, an ER can set working days and hours. The second part is whether you are to be paid or not. If you are non-exempt under the FLSA then you generally must be paid. If you are exempt then you generally wouldn't be paid.