I'm trying to use the ANSI ``stringizing'' preprocessing operator `#' to insert the value of a symbolic constant into a message, but it keeps stringizing the macro's name rather than its value.

It turns out that the definition of # says that it's supposed to stringize a macro argument immediately, without further expanding it (if the argument happens to be the name of another macro). You can use something like the following two-step procedure to force a macro to be expanded as well as stringized:
#define Str(x) #x
#define Xstr(x) Str(x)
#define OP plus
char *opname = Xstr(OP);

This code sets opname to "plus" rather than "OP". (It works because the Xstr() macro expands its argument, and then Str() stringizes it.)
An equivalent circumlocution is necessary with the token-pasting operator ## when the values (rather than the names) of two macros are to be concatenated.
Note that both # and ## operate only during preprocessor macro expansion. You cannot use them in normal source code, but only in macro definitions.