The scanf function uses pointers (aaaaaarrrgggghhhh)
try:
printf("\nType number or name: ");
scanf("%d", &value);
if (isdigit(value))
printf("\nNumber");
else
printf("\nLetter");
I'm not sure if this'll make any difference, but I'd use %c in the scanf to read in a char, instead of %d to read in a decimal integer - although that could just be my preference rather than any C rule.