Try to get hold of the 'Advanced Analyis' library for labview. It contains VIs for doing FFT! No need to reinvent the wheel.
If you can't get it and want to take the hard way, try to learn CIN programming by
- reading the manual (no joke or cynism intended)
- look for advice in NI's developer zone (www.ni.com/devzone) by using the search phrase 'CIN tutorial': you'll get several links which should be helpful in your case.
- better than a CIN: try to use the DLL calling mechanisms and compile your C code into a dll. In order to learn dll intewrfacing for LabVIEW, follow the route explained under point 1 and 2 above...
Good luck,
Franz