The compiler is being as explicit as possible: you're defining a function where it is not allowed to define a function! Here is a snippet
Code:

int main(void)
{
int done = 0;
char choice;
while ( !done )
{
cout << "Menu"
"[1] Area of Circle"
"[2] Convert lbs to kgs"
"[3] Convert mph to fps"
"[4] Coin flip"
"[5] Roll dice"
"Enter choice (q to quit)? ";
cin >> choice;
switch(choice)
{
case '1':
{
int r;
cout << "Enter positive int radius? ";
cin >> r;
double areaOfCircle (int r)
{
double pi = 3.14159;
double a;
a = pi * r * r;
return a;
}
cout << areaOfCircle(r) << "units squared" << endl;
break;
}
//.. rest of 'main' here
}

You're defining this function inside of the function "main". Move the definition outside of this function, and simply call the function from "main". So it may look like:
Code:

double areaOfCircle (int r)
{
double pi = 3.14159;
double a;
a = pi * r * r;
return a;
}
int main(void)
{
int done = 0;
char choice;
while ( !done )
{
cout << "Menu"
"[1] Area of Circle"
"[2] Convert lbs to kgs"
"[3] Convert mph to fps"
"[4] Coin flip"
"[5] Roll dice"
"Enter choice (q to quit)? ";
cin >> choice;
switch(choice)
{
case '1':
{
int r;
cout << "Enter positive int radius? ";
cin >> r;
cout << areaOfCircle(r) << "units squared" << endl;
break;
}
//.. rest of 'main' here
}