The PULSIN command is not too reliable. I have tried something similar to the PWM measurements that you are trying to do with 2 resistors in series and a 100 nF capacitor using ADC in the PIC. Connect 2 resistors in series to ground from the PWM output from the sensor. You can try 100 kOhms both. Connect the ADC input of the PIC to the junction between the two resistors. Conn0ect a 100pF cap to ground from the ADC input port. Get a reading. Try different values for the resistors. It works. Wait like small pause fefore the ADC reading. Something like 20 uSeconds.