The pull-up resistors are not needed externally, they are built in and activated in the ATMega2560. So your mechanical endstops are simply switches between Signal and Ground.
BTW the Ramps is IMHO quite dangerous concerning the endstop pin arrangement, if you put the endstop connectors the wrong way, you will short 5V to GND - and probably damage the 7805 on the Arduino Mega. So, beware of how you put the endstop connectors... Or even better, insert a small resistor (e.g. 270 Ohm) in series with the endstop switch, so even if you put connectors the wrong way, it won't do any harm.