so you have downloaded the binary firmware version 3.11NA, filename = dir615_revC_firmware_311NA.bin correct? Then log into the router 192.168.0.1 and log in, then go to Tools, then on the left menu Firmware? Then click browse, then browse to the file and click upload? You are using a wired connection? If you did all that, try using the reset button, hold it in with a ball point pen for 10 seconds, then unplug for 2 minutes, then plug it back in and try to update it. It definitely needs the latest FW.
If all THAT didn't work, try f/w 3.10, then try 3.11. Other than that, might be faulty?