The gnome-shell applet seems to be incompatible with Gnome 3.8.
Simply adding "3.8" to metadata.json does not work, it seems there has to be done a little bit more.

The applet (small window with the next pause counters) works under Ubuntu Gnome 13.04 with Gnome 3.8.3. I'm using Gnome 3.8.3 staging from the official Gnome-Team PPA and the Workrave version 1.10.1.2-ppa1-raring1 from the workrave PPA (Installed packages workrave, workrave-data and workrave-gnome).

The applet is not officially supporting Gnome 3.8.3 (latest compatible version is 3.6.2) and it's also mentioned to be not compatible in the shell extensions list. However the whole program seems to work anyway. It's even completly in german now, so I'm very comfortable with it.

Yes, the small window works fine. But this is not what I mean by "gnome-shell applet".
It would be great if the applet (https://github.com/rcaelers/workrave/tree/master/frontend/applets/gnome-shell) would be ported to gnome-shell 3.8, as this would have a much better integration into the shell than the extra window provides.

Sébastien Villemot fixed compatibility with 3.8.

(git commit 15f09d8a626e0c1251adc842e5cc5ed05a4cb690)

I fixed compatibility with 3.10.

(git commit 4dc0d1c85e253cf7faa156633cc2deee6a1a92a6)

The gnome shell people have the habit of making backward incompatible API changes each release it seems....