Ok so it seems the defaults are bad, and you have to configure them to be of any use.
I used the most recent version from
http://ts3.cosaofficial.pl/opentrack/ (mine says it's 2.0rc1 too).
Apart from the usual stuff - selecting the right tracker, FreeTrack 2.0 protocol, Accela filter, what needs to be done: click the "Mapping" button, and in the upper graphs of all axes (Yaw/Pitch/Roll/X/Y/Z) add a point on the rightmost side, moving it up into the corner:
Once this is done, you should see the output values (near the pink octopus) working.
Then define a shortcut key for resetting the origin by clicking the "Keys" button, and define shortcut for "Center". Use the shortcut to set the normal head position, you should see the output values reset to zero.
Save the profile and it should work. You may need to adjust the axes sensitivity by modifying the profile curves though.