You should may have to log out, or restart gnome-panel before being able to use your new applet.

You should test that the applet is functionning with the following command:
$ /usr/bin/service-discovery-applet -window
before trying to put it in your panel

To add actions when you click on a specified service, add a file in $(prefix)/share/service-discovery-applet/scripts, you must name it type.sh (eg: _ssh._tcp.sh), look at the included scripts to know how to access ADDRESS, PORT and TXT records.

To add icons for services in the applet, just drop a png named TYPE.png (eg: _ssh._tcp.png) in $(prefix)/share/service-discovery-applet/icons

All of this will be configurable from the GUI in the upcoming releases.