I also just bought a new dir-615 to replace my old linksys router... i followed the set up instructions and i can connect to the router but not the internet, it cant find my modem(Motorola cable modem)/internet connection.
I checked all the connections/cables and their fine so i called my ISP and they said the modem is working fine. I reconnected my linksys and it worked fine.
I tried the release and renew functions while the dlink router was connected but that didn't seem to work.