There are 2 ways to setup Openvpn in Tomato, either way you will need to install is to install the openVPN modded version of the tomato firmware, then set it up from the tomato admin screen. There are many ways to setup Openvpn, in this tutorial we will set it up with a single static key. This setup is recommended if you only need 1 user to connect to your home network to access things locally, or just to hide your traffic when you surf on unsecured channel.
I will assume that you have a router that is tomato/dd-wrt/openwrt enabled, with a variation of the firmware installed, that way it is very easy to flash to the tomato openvpn enabled firmware. Your best friend to this information is google.
First download the firmware.
http://tomatovpn.keithmoyer.com/you can download the binaries, and the latest update is 1.25vpn3.3 release as of this article.
Then go to your tomato router and flash it with the firmware from the Administration Screen.

Notice that I am already using the OpenVpn Modded Tomato, I have attached that screen so users can really see what they are doing.
After you have loaded the firmware and rebooted the router you will see the VPN Tunneling Option in your menu. What you need now is to download openVpn and generate a key, a good tutorial would be to read the materials in Openvpn's main page.
Click on the VPN tunneling option in your router menu. And you will be presented with the following screen

Then you can select the following
Interface Type:TAP
Protocol:UDP
Port:1195
Firewall Custom
Authorization Mode:Static Key
Ignore advanced and goto keys, and you will reach the following screen

Insert the key with the static key you have generated in your copy of OpenVpn. Remember not to share this key with anyone.
Then you should go to your firewall and forward the port 1195 to your router's IP address. In my case, I forward external UDP port 1195 local port 1195 at my router's IP address which is 192.168.1.1
If you do not have static ip with your ISP, it is easier if you configure a DDNS host. You can join free service in either Dyndns.org or no-ip.com. Then you can connect from anywhere to your
xxx.dyndns.org
After that you should configure your local config file for openvpn and save the settings to a configuration file, in our example we name it connect.ovpn.
# Use the following to have your client computer send all traffic through your router
# (remote gateway)
remote
replace this with your server's address or xxx.dyndns.org port 1195
dev tap
secret static.key
proto udp
comp-lzo
route-gateway 192.168.1.1
redirect-gateway
float
Then place your static key in a file in the same directory as your connect.ovpn, make sure the name of the file is "static.key".
You can now connect to your host by right-clicking on your connect.ovpn and select connect option