Are you talking about setting the IP of the BL4S100, or of your computer? @Campbell's answer below explains setting it on the Rabbit, but it sounds like you might also need to set it on your computer.
For Windows, go into the network settings, find your Ethernet interface, and configure it for a static IP instead of DHCP (the default setting). This assumes you're using Wi-Fi for your network connection, and connecting the Rabbit directly to the PC's Ethernet port.