maybe this is helpful for others.
I had this RF modules sent this morning and finally (evening) got them working.
the problem was that i had no comunication between the module on the computer (connected thru a USB to Serial board) and that on the Arduino. The Usb-Serial was in bundle with the two modules, and i've taken them here: http://www.droboticsonline.com/index.php/apc220-radion-communication-module.html
BUT in the site, nor anywhere you can find this article, you are told about an issue with linux.
in fact the enable pin needs a voltage less than 1V or more than 3,5V, but with linux and mac you have just 3.45 on this pin, so the board isn't enabled.
to make the board work on linux or mac one solution is to make a little breakout board that just exclude the enable pin. This way the board is enabled and works ok!
if it was already known, i'm sorry, i didn't find this on this forum.
wish it's helpful for others.