You either have to edit the file when amuled is not running, or reload shared files afterwards. You won't see them as shared in remote-gui's directory tree browser, that's yet unimplemented.
Note, that adding /mnt/VIDEO to shareddir.dat will only share files under /mnt/VIDEO, not its subdirectories. If that directory contains subdirectories, those must also be listed in shreddir.dat.
You can easily create the full list of direcories into shareddir.dat by running "find /mnt/VIDEO -type d -print >> shareddir.dat" (without the quotes).