To embed video in a ScreenSteps step for export to Mindtouch you need to use a media tag rather than embed code. I'm going to use a url to a Viddler video as an example. The url to my video is:

http://www.viddler.com/explore/bluemango/videos/15/

The media tag for this video would look like this:

{{ media("http://www.viddler.com/explore/bluemango/videos/15/", 437, 370) }}

Copy the media tag code to your system clipboard.

Now that the media code is on the clipboard you need to attach it to a step. Right-click on a step in ScreenSteps and select Set Video Embed Code...

Paste in the embed code into the HTML For Video (1) field and click OK (2).

When you export your lesson to Mindtouch it will appear in place of a step image.