1) Log into the NVG510 and go to the "Home Network" tab. Look for 2 lines that say "DHCPv4 Start Address" and "DHCPv4 End Address"
2) Pick a number just OUTSIDE that range. I.e. if "DHCPv4 Start Address" is 192.168.1.50 and "DHCPv4 End Address" is 192.168.1.100, pick a number either 2-49 or 101-253. For the rest of this example I'm going to assume you picked "2"
3) Set the IP on the Slingbox to the one you just picked; i.e. 192.168.1.2
4) Once the Slingbox reboots/accepts the change, reboot the NVG510.
5) As it boots up, the NVG510 should scan the entire network and the Slingbox should appear in the "Needed by Device" drop-down list.
If THAT doesn't work, then I'm out of ideas