When your client device (a PC, laptop, PDA, or smartphone) and your Orb PC are in the same LAN (i.e., both access the Internet via the same public IP address), Orb will always stream your media directly from your Orb PC to that client device without exiting to the Internet.
So you do not need to configure anything in Orb to do LAN streaming.
The main circumstances which would cause Orb to send your stream to the Internet and then back to your device inside your local network are:
you have two separate public IP addresses for your single LAN and your Orb PC's IP is different from that of your client device. Note that in such case, your Orb PC and your client device are not considered as being in the same LAN;
you have a proxy server inside your LAN though which your Orb PC or your Orb client device connects to access the Internet;
your Internet connection goes through a "transparent proxy" set up by your ISP.
In any of the above cases, Orb cannot know nor assume that the IPs of your Orb PC and your Orb client device belong to the same network and, therefore, will stream through Orb servers.