The main problem people run into with the SB600 and D7000 pop-up flash is that they go into the D7000 menu and set the pop-up to Commander, get the settings configured, and forget to press the “OK” button at the end thereby canceling all of the changes.
Set the SB600 as Group A, channel 3, and then set the pop-up flash as Commander (menu e3) on channel 3, with Group A set for “TTL.” Set the Built-In flash to Mode “–” if you don’t want it to show up in the image as well.
Make sure the sensor on the flash is pointed in a useful direction. It’s the circular “window” on the side of the flash next to the battery compartment. Useful range is 30-50 feet depending on lighting conditions (sunlight directly on the sensor window is a range-killer.)
In many cases, aiming the sensor at the subject will get you more than enough bounced signal for the SB600 to fire under the D7000’s control.