You gotta be more specific.
SlingBox has 2 IP
Pubblic IP on the WAN interface and private IP on the LAN interface.
Probably you are talking about the private (LAN) IP.
The Private IP is automatically released to the device when you connect it to your home network by a DHCP.
But if in your home network (LAN) you do not have a DHCP server (a device that give an IP to every other device connected to the LAN requesting it) then you need to set up (to manually give) a specific static (fixed) IP yourself when you configure the slingbox for the first time.
Try to set up it manually...