this is probably a naive question, but I am trying to implement Mathlink like code (I need to run some C code that is talking to some devices to collect data) with Mathematica 9.0. I was searching for the WSTP folder in Mathematica 9.0 installation but I could not find it.
Does anybody know if this is a feature that is not available in Mathematica 9?
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.