HDMI-CEC (REGZA) missing contextmenu button

Hello,
I am controlling XBMC in Openelec on Raspberry B using Toshibas HDMI-CEC implementation called REGZA-LINK. But the "context menu" aka "options button" (= "C" on keyboard) does not work at all; I read somewhere that by default it should be mapped to the GUIDE button of the remote control. Does anybody know how to deal with that problem? I am using Frodo version with libCEC 2.1.1.

Thanks for any help.