Did you follow the instructions here?It pretty simple just dl the firmware and save it somewhere on your pc then go to the upgrade firmware tab,click browse then upload it.But make sure to set it(router)back to defaults before doing so.And it can be done wirelessly but it's better to do it hardwired to the router.
http://www.dd-wrt.com/wiki/index.php...loading_DD-WRT
http://www.dd-wrt.com/wiki/index.php...loading_DD-WRT