Slidebean works online, so videos can be linked from online platforms like Youtube, Vimeo, and Wistia. Uploading video files may be possible in the mid-near future, stay tuned.

To add video, go to "Insert" in the upper menu bar of the editor and select Media, then Video. 

You'll get a field to just paste the video URL from the platforms mentioned above.
You can manually resize and reposition the video, along with the rest of the elements in the slide.