Layer-2 Bridging is very simple to setup.In order for the Access Server to be able to assign an IP address to the clients you will need to make sure you have a DHCP server that resides on the same network as the OpenVPN Access Server.
The first step you want to take after logging into the Admin UI is to navigate to the "VPN Mode Page".
Now that you are at the VPN Mode page in the Admin UI you are going to need to Select the Option that Says: Layer 2 (ethernet bridging):
That's it. Bridging will now work, keep in mind that a DHCP server needs to sit on the network that your OpenVPN Access Server is on in order to Assign IP addresses to the VPN clients.