I quickly build up the circuit with the bits I have.
From left to right: I used a USB power bit, added a fork bit to have the chance to use more bits to be connected to the Arduino. I connected the rain sensor circuit to the proto bit with jumper wires. Additionally a dimmer bits has been added to adjust the trigger level for "It's raining" indicator.
On the right side I have connected my wireless transmitter bit, but you could also connect a cloud bit or any other output bit.
Please note that you have to remove the middle jumper of the proto bit.
Of course there are other ways to connect the rain sensor to the littleBits universe, but this is the most convenient way.
Here is a summary of what I've done.