I guess watch them all and see what ones do not have an reference to MediaSignage. I saw what you mean and can understand what happened to you. That sucks. Though the videos are a great idea, the problem with them is that most are outdated as far as the interface even though the content is pretty much unchanged. It can confuse someone who doesn't know that there were older versions and looking at an interface that looks nothing like 4.0.
Best bet for now, until new ones are created, would probably to create some yourself. This way you always have updated material that is tailored to you and you could even place your logo on them. It is not very hard to do at all. Just takes some time. This will also make you better understand the system yourself so that you can provide awesome customer service to your clients.
