I had a little trouble with this, in the manual it says to pop the disc in, which I thought could be ignored as I already had the client. However it still wasn't showing up on the mogstation. So I popped the disc in, it did a little update then it asked me to log in, once I did that I followed the on screen instructions to link it to my service account and it's now showing up as registered through the mogstation.

Hope this helps you somewhat