okay, theres a few things that i can see wrong with your code.
Line 7
void mrinch ( double inches[]);
your declaring the function "mrinch" ,but the function doesnt exist within your code.
Line 37
double calculateInches(double inches)
your defining the function "calculateInches" but the function hasnt been declared at the top.
maybe your declaration on line 7 should be
double calculateInches(double inches);
in your scanf calls your always reading the data into the same place so it will overwrite the data each time. the number in the square brackets is the position in the array so in your case between 0(first number) - 4(last number)
your code
printf("Please enter value number 1 in inches: ");
scanf("%lf", &myInchesArray[2] );
should be
printf("Please enter value number 1 in inches: ");
scanf("%lf", &myInchesArray[0] );
and another possible implementation:
Give a man a fish and he will eat for a day, teach a man to fish and he will get tired of eating fish.