Grumpy_Mike, thanks for showing the picture. I yet don't know how to do that in this forum. 
Olf2012: I read tutorial 148; Fig 1 comes from it. 
Fig 2 is the electric schema I implemented for Arduino based on Fig 1.
R1 is a resistor 1kohm; R2 is the 3.3kohm, 220pF is the capacitor and the three-pin thing between ground and Vcc is the 2N7000. Using a Mega you have ground on PIN 55 and Vcc on PIN 22. 1-Wire is on PIN 49.
I am not sure this implementation is the right one.
From Fig 1 I cannot understand where I should connect R2 to. I assume that what in Fig 1 is called 1-wire Network is the connection to the PIN I defined in the Arduino instruction OneWire (e.g. OneWire ow(49)). Ground is ground. But where is the wire to R2 coming from? In Fig 1 it comes from a generic Microcontroller box, but where should I connect this wire to Arduino? Ground, Vcc or where?
Thanks.