JuanB,
Just want to let you know I appreciate your fine work on making this library available.
I am using the Eclipse IDE with the Arduino 1.5.2 toolchain.
I downloaded the version 9 of Modbus Slave and have it seemingly working fine on a Ether Mega 2560 board.
I edited out the serial parity sections for it to compile correctly under the 1.5.2 toolchain.
Since I'm on Mac OSX and Linux I wanted to test it, so I downloaded a GUI based Modbus master program which will only operate under Windows unfortunately, this one http://www.protoconvert.com/SoftwareTools/Modbus/ModbusMasterSimulator.aspx
So I ran that within VirtualBox which I have Windows XP and set up the COM port to be directed back to the USB port to which the Arduino Ether Mega (Freetronics) is connected to.
Well, blow me down, it works, albit with some time-outs 
I might even try making a small SCADA test using CitectSCADA I have on my XP image.
(My aim is to lessen my dependance expensive Allen Bradley Control/Compact Logix PLCs in my future projects, and to use start using Modbus with a mixture of Arduino based controllers together PLCs that can talk Modbus)
Next is to write some nice Python code to have my Cubieboard talk Modbus to serial1 on the Ether Mega.
Then the Cubieboard is my local server and link back to the outside world.
So, yes, your code works on Arduino Mega under 1.5.2 (Eclipse IDE), keep up the good work
Paul (Tasmania)