ethernet2 is a completely different library, so it needs its own folder in the libraries folder. You shouldnt put ethernet2 stuff in the ethernet folder. also, you have to change the #include <ethernet.h> to #include <ethernet2.h> and then original client should work fine. Also, if you want to make it really easy on yourself, the entire ethernet2 folder is all nicely zipped up so all you have to do is unzip it into your hardware->libraries folder. its in this post: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1233499187
/Applications/arduino-0017/Arduino.app/Contents/Resources/Java/hardware/libraries/Ethernet2/Server.cpp:75: error: 'EthernetClass' has not been declared