Hi Galina,
I will look into your problem with Muscle Max, but I do need to know the length of the workout that you are trying to create in order to let you know about how long it should take to generate the workout. When creating a long workout, the progress bar does sometimes seem to be "frozen" at 80% or 90%, but this is because at that point in the workout's creation is when re-encoding of the video is happening. Depending on the video, it can take a while.
Also, we do not have a "Cancel" button yet, but quitting the application does serve to quit the creation of the video as a workaround for now.
Thanks!