This is quite common with this sensor and Pduino. In the code one can find for this sensor you can see that it uses the Arduino native pulseIn() function, which is called when the echo pin sends a signal to the Arduino. In order to use this sensor with Pduino you need to translate the workings of this function to a Pd patch.
Instead you can just tweak the printing functions of the code and use the [serial_print] abstraction that I've made, especially for situations like this. You can find it here https://github.com/alexdrymonitis/Arduino_Pd
All you need to do is make sure you first print some sort of tag with Serial.print() (like "sensVal" for example), and the print the value with Serial.println(). Make sure you print a space in between the two. So you can do the following:
In the example above the sensor values are being stored in a variable called sensVal. I first print the string "sensVal " (note the white space before the closing quote) and then I print the variable (no quotes this time) with Serial.println().
Then in Pd follow the example of the help patch of [serial_print] and use a [r sensVal] to receive the values.
This should do it.