Linksys WRT610N v2.0
From DD-WRT Wiki
Installation
NOTE: During configuration or flashing a device, the only cables that should be attached to the device are the network cable to your computer and the power cable.
Warning: This unit MUST use K2.6 firmware.
Make sure you use a firmware with wrt610nv2 in the filename like the one below.
ftp://dd-wrt.com/others/eko/V24-K26/svn13575/dd-wrt.v24-13575_NEWD-2_K2.6_mini_wrt610nv2.bin
a. Flash firmware through the Linksys WebGUI.
b. After a couple minutes you should see an "Update Completed Succesfully" Screen.
c. Wait...at least three minutes after that just to be safe.
d. Do a power cycle of the router. (Unplug the cord, count to 10 and plug it back in.)
e. Wait for the lights to return to normal, usually about 2 minutes.
f. Do a HARD reset. Wait. Check for the password page and login to change the password.
g. After you have DD-WRT on the router then you can flash a different firmware from this folder through the DD-WRT WebGUI.
ftp://dd-wrt.com/others/eko/V24-K26/svn13575/
Make sure that you do not flash any build to your router that has the name of a different model router at the end of the filename.