Run "rosmake", and then "cd build && make upload" to upload it to the arduino. You also have to "rosrun rosserial_python serial_node.py ..." with some arguments for the port the arduino is connected to. If you are using the micro: In order to make this successfully compile for Leonardo, you will need to copy ArduinoHardware.h to cd `rospack find rosserial_arduino`/src/ros_lib, and save a backup of the one that is in that folder. The header uses pointers to HardwareSerial objects, but for Leonardo it's called Serial_.