You can use
Media Downloader - I just tested it myself. To install it, scroll down to the section 'Binary Packages', where there will be links for your system.
Once you've installed it, click Options, and choose Best Available Audio Video. Then, paste the Nico Nico Douga link into the URL box. The video should appear in your downloads folder, if you haven't changed the default installation path. It may be dated to the upload date of the original video, so it might not appear at the top of your list of files.