That pad connects to the same place as the circuit used for resetting the ATMega328P chip - that is, pin D7 on the ATMega8U2.
It just pulls the pin PD7 down to ground with more "force" than the internal pullup of the pin itself.
You can attach that resistor to anywhere along that line, and probably the most convenient place would be to the ATMega16U2 side of capacitor C5.
That capacitor has one end linked to the RESET pin on the main Arduino header, so you can connect a DMM in continuity test to that pin and "buzz out" each capacitor until you find the one point where it beeps (I think it's one that's right next to "Reset EN").
Carefully solder your resistor to the opposite side of the capacitor to the side that beeps (the side nearest the ATMega8U2 chip, probably), and connect the other end of the resistor to GND on one of the main headers.
By the way - get yourself a flux pen and apply flux to where you are soldering before you start soldering. It should help you avoid these kind of problems in the future. Also get a proper temperature controlled soldering iron (I use 350C for mine most of the time) and some solder wick to help recover from solder messes. All part of the standard EE's toolkit.