You have incorrectly specified a KeySym (mnemonic).
Unfortunately, the correct specification of valid KeySym mnemonics remains undocumented.
The definition of KeySyms is normally located in the header file /usr/openwin/include/keysymdef.h. Type in to the Mnemonic field the keysym minus the XK_ prefix.
For example, if you want the Mnemonic for C++ to be C, then type C. The valid KeySym for + is plus.