If that router was supplied to you by your ISP you will need to contact them for instructions on updating the DNS settings, as they may have loaded their own, proprietary firmware on the device. Alternatively, you can always change the DNS settings on your computer instead (see https://store.opendns.com/setup/computer/). You will get the same benefits, and if you have a small number of computers on your network, it's relatively easy. In some cases, users have found success in configuring both their router and computers.
