First, click on File / OSC Settings to go to the OSC panel. Now scroll down to the second screenshot below for exact settings.
To get Reaktor sending messages to the Lemur, you need to create and select an entry in the middle part, where it says Lemur in this screenshot. Make sure the IP address is correct, select port 8000 and then click Apply. In the Lemur app, make sure that OSC Target 0 is pointing to the IP address of your computer on port 8000.
The kind folks at Twisted Tools have created a nice PDF guide going into this process in greater detail: