You can download a video from web pages which contain video. So, firstly, please, check if you are on a web-page which contains a video and this video is playing in your browser.
If a video is detected and ready to be downloaded you will see the 'Download' button active and notice box popping out to let you know that video is ready to be downloaded.
If the button is not activated you might have to click the Play button in the video in your browser first, so it starts to play.
If it still doesn't work, try closing the page, open it again, play the video, then click the Download button from our Video Downloader.