You first need to install a dvd-ram driver to use the dvd-ram discs for packet writing. That way, you can add and remove files from within Windows Explorer.
The easiest solution: use InCD from Ahead (which is usually included with Nero). Otherwise, use the Panasonic dvd-ram driver. You can search for it on this forum, this question has been asked and asnwered many times before.
If you use a normal burning program, you can use multisession to add more data later, but that is not the best way to use dvd-ram.