What a coincidence, we have been talking about that in code.trisquel
recently.
What it is happening is that there are no "tray" but an "indicator applet"
instead. Initially, the tray was alongside the indicator applet but it was
later remove because there was some bug in the panel. now the problem seems
solved and we are discussing to bring it back (see merge request #110).
To add the tray to the applet: "alt" + "righ click" on the panel and select
to add applets. Then choose the "notification area", and be sure it is on the
panel. It is initially empty so try to start the program and see if it is
there, if not, be sure you put the applet correctly.
I hope it helped.