"So there's the problem -- improper use of the 'data' keyword."

Yep, that would certainly be my first guess.

However, I'm suprised that you only got the syntax error in main(), rather than in definition of the structure; using the 'pre' and '/pre' HTML tags to make it legible:

struct 
{ 
   char data_id; 
   union 
   { 
      char c; 
      int i; 
   } data; // is 'data' allowed here??
} message; 

void main (void) 
{ 
... 
message.data_id = 0; // This works 
message.data.c = 0;  // => Syntax error (It doesn't compile) 
... 
}

Or have we not quite been told the whole story?