The DD-WRT disables the Cisco Light. But some people like it on, here is how.
Note: I don't think this has any "if then" statements like it usually would, IE: If Wi-Fi was down it would normally blink, I'm *NOT SURE* but I don't think this will happen via this command. Read LED DD-WRT Scripts for more information and options.
Go To Administration >> Commands
Then "Run Commands"
Then "Save Startup"
If it doesn't turn on right away, reboot via Management (Scroll to bottom) >> Reboot Router.