Find the network settings applet on your Mac and double check the Default Gateway address, that is the IP you need to connect to to access the router's management interface.
If it is 192.168.1.1, then reboot the router (let it sit powered off for 5 to 10 seconds) and then try again.
If you still can't get into it, you will likley have to try a factory reset.
If that doesn't do the trick, do yourself a favor and stay away from DD-WRT and try one of the better 3'd party software like my personal favorite, Tomato If tomato isn't available for your make/model router, better to leave it with the default than use DD-WRT.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.