Connecting RJ11 plug output to Arduino
The rain collector appears to have only a reed switch contact closure output. The collector itself will not produce voltages on the wires. Instead you have to connect the reed switch contact wires into a circuit that puts a bias on the wires that can be detected by the Arduino. 

Since the wires are most likely running from outdoors (wet rain location) to indoors (dry Arduino location) it is a good idea to prepare some additional protection circuitry before the input to the Arduino so that if the wires experience some electrical disturbance it does not knock out your MCU board. You could try the following circuit as a starting place to hook up the rain collector reed switch.