It is called MathLink, not WSTP, in version 9 and earlier. In version 10 it has been renamed to WSTP and all ML-prefixed functions have been renamed to WS-prefix functions. Yes, the naming of the API functions has been changed for marketing reasons.
If you need compatibility with earlier versions, use the ML-prefix names.
http://reference.wolfram.com/legacy/v9/tutorial/SettingUpExternalFunctionsToBeCalledFromMathematica.html
