Then give up and use my instructions (For the very specific case of a ddwrt router behind a westell dsl modem):
- find out your modem's ip address by checking the ddwrt status page for the WAN gateway.
- open the modem ip in your browser and navigate to Status->"Connection Summary"
- copy down the dns servers (188.8.131.52 and 184.108.40.206 for me). If they don't end in .12 these instructions are probably outdated and invalid.
- back on the dd-wrt interface, go to Setup->"Basic Setup" and change your connection type to "Static IP"
- Leave everything alone except the DNS settings that are now showing. These are the numbers from step 3 with .14 on the end instead of .12