Why do you need the user to type Start to start the program? Wasn't running it enough to convince you that they wanted it to run? (I point this out because I absolutely hate "are you sure"-style messages. Yes I'm sure, otherwise I wouldn't have typed that command would I.)
I guess your errors are along the lines of "conamt not defined" and "amt not defined", and you're looking at main and thinking "yes they are defined". They're not defined for class currencies, hence the errors. Variables defined in a function are effectively "private" to that function and nobody else has access, except where you pass pointers to the variables, or where you pass the variables' values to called functions.
So here's my suggestion: define amt and conamt (if needed) in the class, then just call functions in the class from main. Have a member function that prompts for input, and a function that prompts for the currency to convert to, then depeding on the input given either call a function to do the conversion directly, or pass something back to main so that it can call the conversion function. It's up to you if the conversion function displays the output or leaves it to main to call a function that displays the output.