there should be an unused servo plug at your gyromixer (it should provide in total 5 servo plugs), if Luv used the same color scheme as I do it would be marked with a yellow shrink tube piece, but I can't know of course.
Anyhow, the RobbeBox just goes into this additional plug... BUT BE CAREFULL: connect it correctly, the GND line should be at the top
The RobbeBox will display a message, which usually will be "disconnected". If you power up your system with the RobbeBox plugged in it will connect and you can go through the menu. Also, after power up, you can always establish a connection after the throttle has been totally low for more than 2 seconds (you may have noted that if you land and wait for few seconds the red led will flash, which will go away if you start again, the red led tells that the gyromixer is in what I call the THROTTLE LOW state). That is, fly around, then land and keep throttle low, plug in the RobbeBox and wait until connection is establish, do the changes you want, disconnect the RibbeBox, fly around and so on. BTW: while connected with the RobbeBox, the gyromixer will flash the blue led.
You can find more and hopefully sufficiently detail info also in post #6 and #7 of the gyromixer thread: here