The problem is that you only ever initialise the value of one element of your array. You should scan each value in turn inside your loop, ensuring that your loop ends when i is equal to MAX_ITEM. You also need to flush the input stream after each number, to clear the line end characters. So you should end up with something like:
int i;
sum = 0.0; for (i = 0; i < MAX_ITEM; i++) {
printf("enter a number: ");
scanf_s("%lf", &x[i]); fflush(stdin); sum += x[i]; }
printf("sum = %lf \n", sum);
As you can see the array x is not really necessary in this case as it is never used after reading a value. A single variable would do the job.