To download a video from Kaltura it has to be your own. You are not allowed to download a video from a different user unless they share that video with you.
To start off you need to make sure that you are logged in. In the top right hand corner it will say guest if you are not logged in.
Click on that button and then click on log in.
The best place to download a video from is My Media, but you can also find it on your channel. To find these areas click on your name in the right hand corner.
Once you are in the My Media area or on one of your channels click on the edit button.
Click on Downloads
You will be given the option of these different formats.
The best choice will always be source as it will not degrade the quality of your video.