userfriendly, do "ipconfig /all" at the command prompt (before doing it be sure that ur connected in the network). Look into the LOcal Area Connection, write down the IP address there (example: 169.254.1.100), now open ur browser type in 169.254.1.1 (just copy the whole thing and replace the last digit with 1) then Enter. You should be able to access na your config.
The config address changes according to the IP address range that u specified in the router (192.168.1.100; 169.254.1.20), just remember to replace the last digit into 1.
FYI: The default is 192.168.1.1 for Linksys, 192.168.0.1 for D-Link; but if you change the IP range in the router they follow ur IP range.
Another, based on your comments I think your using a wireless laptop to access ur config. It is also possible that in your router config, u could have disabled the "COnfig Access through Wireless" it means u have to be wired to access the config/setup.