asuming you opened the Marlin.pde file in the Arduino IDE, then you just click on the right tab. You might have to scroll to the right quite a bit, but it will be there.
Or you go to your Marlin directory and open it manualy with a Texteditor.
It looks like you have a recent build of the Marlin firmware, which did away with boards.h and the motherboard numbers (i.e. 33 = RAMPS1.4), in favor of names.
You should use #define MOTHERBOARD board_ramps_13_EFB for a RAMPS 1.3 or higher with D8 connected to the heated bed, D9 connected to the print cooling fan, and D10 connected to the extruder heater.
