If your DD-WRT does not show any advanced configuration options, you may need to either upgrade to a big/mega build or use another firmware such as Tomato or PfSense.
Please always use caution when flashing a custom ROM to your router. We are not responsible for damaged routers. It may void your warranty.
Another alternative is to use the following set-up from one of our community members:
This is a script for configuring OpenVPN on DD-WRT, and should work even if your OpenVPN client is lacking some of the necessary settings. Please note that your router still requires some form of OpenVPN client be included in its firmware for this script to work.