In both cases use the "Download Zip" button on the right.
For some spec. boards a spec. dir in the ArduinoAddons directory in the Marlin dir needs to be copied to the arduino environment. <arduinohome>\hardware\
Start the arduino IDE.
Start the arduino IDE.
Select Tools -> Board -> Arduino Mega 2560 or your microcontroller
Select Tools -> Board -> Arduino Mega 2560 or your microcontroller
Select the correct serial port in Tools ->Serial Port
Select the correct serial port in Tools ->Serial Port