To Program the Arduino over a MAX232 one simply needs to press the reset button on the board or to ground the reset pin till the Arduino IDE has compiled the program it is ready to upload. At this point release the reset button or remove the grounding wire and the Arduino IDE will start uploading the program.

I have test this with Arduino UNO and NANO. In the image below we can see the configuration to program an Arduino UNO, the Mega is only used to power the UNO.