Hi Federico
You've done a bit of mix and matching here; there are two TCP/IP stacks around on the site - Rolf's and Donatien's.
You've ended up with code expecting Donatien's libraries, but then linked Rolf's precompiled library to your program. That's why EthernetNetIf.h is not found, it is unique to Donatien's stack.
If you go to Donatien's Twitter example (http://mbed.org/cookbook/Twitter) and then scroll down the page, you'll get to a section that says "You can also import the published program:". Click on TwitterExample which will take you to a new page, then click on "Import into compiler".
That should give you the example with the correct library.
Regards
Daniel
I can't use EthernerNetIf.h, always the same error:
"Cannot open source input file "EthernetNetIf.h": No such file or directory (E5)" in file "/main.cpp"
Any help?