Hi mgvanosd,
Your Arduino needs to be a modbus slave. You don't need a separate device.You can use jpmzometa's mobus rtu slave library in your Arduino. I added some code to make it work with Mango. In the meantime this has been included in the library. You can download it from:
http://sites.google.com/site/jpmzometa/arduino-mbrt/arduino-modbus-slave
Some sample code can be found here: