To edit content you must click on the “wrench + screwdriver” symbol (blue box) first to enter editing mode. You have to do this every time you want to enter edit mode (e.g. if you were before in pen mode). The first time you have to log in unless your credentials were stored by “remember me”. Only then will the editing tools.
Then you click on the add-video button (red box) to open the dialog.
Using this dialog you can upload videos in (.mp4) format. You can also drag videos from your local storage (but not from the browser) into the collection area on the right. Ctrl-V pasting is not support for videos because the clipboard does not hold video data.
SizeHint is explained here.