Here you will allways find at the bottom of this page, the latest binary firmware + GPL sources.
Actually, the latest version available of the firmware is v0.7.5 (29 jan. 2011).
The binary .hex firmware must be extracted to be able to flash the module.
Look at the PDF doc for the avrdude command to use to make it possible !
Verify before in the setup menu which version you have,
perhaps you already have the latest firmware on your module...
An interesting software that you could need for the setup menu, is GtkTerm for Linux:
The latest version can drive the RTS signal in software to select between transmit/receive in RS485 (half-duplex).