char input; // user will type, so input is a char!
 
while (input != 'q' || input != 'Q') // loops untill you put q or Q in input
{
   code here...
   I would use cin >> input;
   then a strtoi (i think that is how it is spelled) call to convert the char to a number
}

#include <stdio.h> 
#include <stdlib.h>
 
/* Main Program */
int main(void)
{
  float USD;
  float CAD;
  float EUR;
  float GBP;
  float CHF;
  float YEN;
  int input;
 
  /*Title of program*/
  printf("\t This program will convert foreign currency to US Dollars\n\n");
 
  /*Assigning Values*/
  USD = 1.0f; /* US Dollar*/
  CAD = 1.2257f; /* Canadian Dollar*/
  EUR = 0.752162f; /* European Euro */
  GBP = .51573f; /* British Pound */
  CHF = 1.134f; /* Swiss Franc*/
  YEN = 1.038f; /* Japanese Yen*/
 
  for (;;) {
    printf("Please choose form the following list of currency or press q to quit.\n"); 
    printf("\n");
    printf("[1] Canadian Dollar\n");
    printf("[2] British Pound\n");
    printf("[3] European Euro\n");
    printf("[4] Swiss Franc\n");
    printf("[5] Japanese Yen\n"); 
    printf("\n");
    if (scanf("%d", &input) != 1)
      break;
 
    /*switching statements */
    switch (input)
    {
    case 1 : 
      printf("%1.6f Canadian Dollars is equal to %1.1f US Dollar.\n", CAD,USD);
      break;
    case 2 :
      printf("%1.6f European Euro is equal to %1.1f US Dollar.\n", EUR,USD); 
      break;
    case 3 :
      printf("%1.6f British Pound is equal to %1.1f US Dollar.\n", GBP,USD);
      break;
    case 4 : 
      printf("%1.6f Swiss Franc is equal to %1.1f US Dollar.\n", CHF,USD);
      break;
    case 5 :
      printf("%1.6f Japanese Yen is equal to %1.1f US Dollar.\n", YEN,USD); 
      break;
    }
  }
 
  return 0;
}