just take the setting that is closest but HIGHER than the value you want to measure. I assume (mine has auotrange) that there is a 1k setting, which would be good for the 680R and the 1M setting should work for the 680k.
resistor codes are blue-grey-black-black-brown (the last brown ring depends on the tolerance though) for 680R, and blue-grey-black-orange-brown for the 680k