What's the best way to fix this missing solder pad?
It looks like that pad (and another one as well) is not connected to any trace on this side. If you can solder it on the other side it should be sufficient.

If you cannot access it on the other side then one simple solution would be to find where that trace goes and solder piece of wire between jack pin(s) and any of the other pads on same net.

This will probably not hold the jack in place though, so some epoxy can be used on the other side to fix it to PCB.