Deskbar-Applet bugs are tracked using GNOME Bugzilla.
Also, please try to test your bug against the latest development version, first. Your problem may already be fixed!
If you want to write your own plugins for Deskbar-Applet read the guide Writing new-style modules for Deskbar-Applet.
All you have to do is go to preferences and drag your module file on the list of modules. That's it! You should then be able to activate your module in Deskbar-Applet's preferences.
$ git clone git://git.gnome.org/deskbar-applet $ cd deskbar-applet $ ./autogen.sh --prefix=/usr $ make # make install (as root)