Follow the instructions on the lets make robots webpage to install the drivers for the USBASP programmer. You will need to disable driver signature protection in order to install the drivers for the USBASP programmer. The USBASP driver includes AVRDUDE.
To test the installation, plug the USBASP into the computer and using the adaptor board plug the flat cable into the mega board. The adapter board pulls down the reset button in order for the boardf to be programmed.
Open a command prompt and enter: avrdude -c usbasp -p atmega128
The response should look like: Note: It is normal for AVRdude to give a warning about the SCK period not being able to be set. It does not interfere with programming the atmega128. It is a known issue with the USBASP programmer driver.
When you are ready to program the board enter the command: avrdude -c usbasp -c atmega128 -U flash:w:filename
Mac:
Follow the instructions on the object development page to install Crosspack which includes AVRdude, avr-gcc and a few other helpful program.
Once successfully installed follow the connection setup from step 2 of the windows setup.