It looks like there are 2 candidates, JP10 and JP12. I'd try JP12 first.
To test, if you don't have a multimeter, connect the GND of the convertor to the chassis, and try the RX of the convertor on either of the 2 centre pins.
With the terminal program (PuTTY works well) set to 115,200 baud, 8 bits no parity, check for output when the DVR is powered on.
When you get that, connect the TX to the other centre pin.
One of the outer pins will be 5v, one will be GND. But I don't know which as I'm away just now. You could just leave GND connected to the chassis if that works.