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and after you unpack it, go to the RTcmix/genlib folder, that's where the Offt.cpp source code is. To use fftw, you need to set up a 'plan' (real-to-complex, complex-to-real) and then execute it on incoming frames of samples (or frames of FFT data). It's fairly easy to use, as fftw provides functions for doing all this.
For info, check out some of the links here under "FFT Information":