Sample rate divided by the number of bins (which is the same as the fft size) times the bin number (starting from 0) will give you the frequency of each bin. Note that bins above the Nyquist frequency (the top 50% of the bins) "fold back" (or more correctly, wrap around into the negative frequencies) and thus essentially duplicate the first half of the bins. MSP tutorials 25 and 26 discuss this further; chapter 25 shows an example specifically addressing your question.

All that aside, you might find it more manageable to use fffb~ instead of fft~ for what you're apparently trying to do.