What do i have to include in a Library to use Serial.println()?
Not in a sketch.
You'll need to run Serial.begin( 9600 ); in the setup()
If you want to use Serial object in a library, you can include the HardwareSerial.h or Print.h if you want to accept SoftSerials and Ethernet connections etcetc.
thanks. I need a bit of Debug output in the library so i have to call Serial.Print in the library - that was my problem.