There are 2 different options for watermarking in MediaLab - 'Video player overlay' and 'Burned in'.
When the 'burned in' option is turned on in the Brand Settings "Formats" table, the watermark will be automatically burned into the chosen versions after uploading. Files that were uploaded earlier can be re-transcoded with the "Transcode" option in the file menu. The original source file remains unchanged.
The 'video player overlay' option can be turned on in the 'Share' form. Then the watermark image is only shown as an overlay in the video player.
Please note that it is not possible to activate both watermark options at the same time, as it results in having two watermarks displayed on the same video.