If your file has been uploaded directly under the main bucket in your Amazon S3 account (same bucket name that you configured in the plugin's settings), then just enter JUST the file name here.
File name may NOT contain spaces, special characters, etc. Hyphens are ok. Like my-facebook-secrets.mp4
Eg: facebooksecrets.mp4
If you have created a sub-folder (or "sub bucket") within your main bucket (that you have entered in the S3MV main settings) then enter it like this... subfolder/filename.ext
Eg: videos/facebooksecrets.mp4
WRONG: Do not enter an entire URL - like http://yourbucket.s3.amazonaws.com/yourfilename.pdf
RIGHT: Enter just the file name: yourfilename.pdf or your-file-name.mp4
Do not use this option for local files that aren't video or audio. Use this option only for locally stored video and audio files. Please note that there will be no protection for such files locally through the S3MediaVault plugin. If you want to protect local files, you have to do it through DAP and take a few extra setup steps, and we don't recommend doing that. For full protection, store your media on Amazon S3, link to them using S3MediaVault, and protect the page on which the S3MediaVault shortcode is published using DAP.
But for a quick and easy video player for your public videos, you are welcome to use this option.
If you have uploaded a video or audio file directly to your site, either via FTP or by uploading it as WordPress media, then enter URL to the file minus the domain name.
So if your file is located at http://yoursite.com/videos/file.mp4, then remove the domain name http://yoursite.com (or http://www.yoursite.com) and enter the rest here, like...
Eg. /videos/file.mp4
You can now use the S3MediaVault plugin to play YouTube videos. Obviously, the video is not going to be fully secure, because you cannot protect a public YouTube video. Even if you make the video as private, it is still not really secure, as the viewer can still click on the YouTube logo that shows up in the bottom of the video player, and go directly to the YouTube page where the video is hosted on YouTube.com. So if you choose this option, make sure it's not for protecting the video, but more so to present a uniform player on your site for all protected S3 videos as well as public YouTube videos.