... but when i try to use pulseIn() with pin 2 like this code i got only 0
The pulseIn() is a blocking function and is not required for this very capable and accurate instrument, as your U1000 has both analog and pulse outputs.
The output is a MOSFET relay type and the electronic normlly-opened contacts are "dry" (high impedance) when open. At the Arduino end, I would connect one end of a 1K resistor to the white wire and the other end to GND. Also, the green wire needs to be connected to GND. You should now get pulses 0-5V on pin 2. EDIT: (as per KenF diagram) These pulses are ideal for totalizing (counting) to record accumulated volume.
The isolated 4-20mA signal can be monitored by using one of the analog inputs of the Arduino. This signal will provide the same information as the digital signal, however you can get accurate and near instantaneous readings which is ideal for monitoring flow rate, data logging, creating flow profiles, etc.
To connect to the Arduino, connect a good quality (temp stable) 250Ω resistor across the red and black wires. Connect the red wire to an analog input and the black wire to GND of the Arduino. This will give a 1-5 volt signal.
Disclaimer: I have never used this device.