It seems that like the instructions on how to use are nowhere to be found, indeed.
Well, here are some basic steps:
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Click the "Add New Download" button in the upper left area
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A popup window will appear asking for a webpage URL
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Enter there the webpage URL of the page from where you want to download video/audio
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Press "Download"
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Once the download is complete, check the download folder (you may right click and choose "Open Download Location") for the downloaded files