need help making a c program that reads flow meter
I have no idea why it seems to you that you should be doing ADC reads to get your value. 

What you need to be doing is counting the pulses from the meter per unit of time. So maybe you should be setting up an interrupt that comes once per second. Also setup a timer/counter channel in counter mode to increment once each time a pulse comes in from the meter. At each second you capture the counter value and re-zero for the counter for the next interval. The count of pulses per second is a direct reading of Hz and can be related to flow rate via the data sheet table.