I was having the same problem and it turns out that you can't use a magnet URL.
So the next best thing is to convert the magnet URL to get a torrent file, which you can then download and add into your DLINK sharecenter.
1. Copy the Magnet URL link by right-clicking on it and then choose "Copy link address" (Note: This instruction is for Chrome browser. You need to look for something similar for other browsers).
2. Go to http://magnet2torrent.com and paste the magnet url into the textbox and you can then download the torrent file.
3. Add the torrent file onto your ShareCenter and you can now download the torrent