It’s really simple to add videos now with the new backend set up. All you have to do is

1. Go down to Custom Fields (below the source box. you do not need to edit the post source box anymore).

2. Select embedvideo from the dropdown, then insert the embed code into value:

3. Click Add Custom Field

4. Select thumbnail from the dropdown, do the same thing except with an image path:

5. Publish as usual, and Voila!