You also need a newer kernel, as stated in the release:
dropline GNOME works with the default Slackware kernel. However, certain features like auto-mounting will not work, unless a 2.6.16.x kernel is installed and in use.
I do have newer kernels here, if you don't care to build yourself:
http://harmon.droplinegnome.org/extra/
(look for kernel-generic, kernel-modules, and kernel-source)
Scott.