In addition to uploading videos to YouTube or Zaption, Basic and Pro users are able to add a web-optimized MP4 file to Zaption lessons.
In order for the video to play back smoothly, videos should be optimized for web streaming. Make sure you compress the video small enough so that it plays smoothly, even on poor connections. Also, the MP4 container header should be at the start of the file, which can often be accomplished via the "Web Optimized" or "Fast Start" option in your encoder. More information on formatting MP4s for the web can be found here.
If the video is embedded, right click the video to copy the video URL.
When creating a New lesson in Zaption, select "browse my videos" from the Add Videos modal. Then select "MP4 URL" from the video channel dropdown menu in this modal. Once this is selected, paste the MP4 video URL into the search box. This will bring up with video and allow you to add it to a lesson.
A test MP4 can be found here.