Hello!
I am wanting to measure the vibration of a machine that I am working on, ideally in frequency. I acquired the ADXL337 and without properly understanding the product, expected it to output g values and hz values.
Is there any way that I can output a frequency value rather than the acceleration value? Is there a way to convert it?
Thanks in advance! 
You can use a computer to determine the vibrational frequencies in the acceleration data (there will be many). The most popular techniques used in the analysis are Fourier Transform and Autocorrelation.
Even a small microprocessor can perform the basic task, but the details depend on your particular application. So, tell us about your project.
The ADXL337 just outputs an analog voltage that’s proportional to how many g’s it’s sensing, it can’t output a frequency value. You’d need a micro controller or other processor to analyze changes in the analog outputs to determine a frequency.