Read the wl500g.dyndns.org page again. It's all there, but requires some Linux knowledge and DYI work. The firmwares from Oleg for the WL-HDD require some setting up for samba it to work directly on the WL-HDD. This is because these firmwares are originally meant for the WL-500(b)(g)(gx) routers.
Another option is to flash an official Asus WL-HDD firmware or a custom firmware from JOKEYW2000. In these firmwares Samba works right out of the box.