Hans,
Grab the online manual from:
http://www.seeedstudio.com/depot/images/product/dsomanual.pdf
Page 9 shows a picture of the earlier keypad labeling. From there you will see that the down arrow key was once a “-” key.
I was much happier after installing the “Paul” firmware found on this forum. There’s also other alternate firmware that just came out - haven’t tried it yet.
It’s noteworthy that the “lanyard hook” is actually where the test signal emerges. Clip your red lead on there to get some immediate results on the screen.
Deveron