Hi - I have one connected to my Avant - so not sure about yours but on mine I have a SKY box connected to one scart as SAT (STB) and on the Apple TV I have it connected as V.AUX (STB) the third socket takes the DVD. When you press ok after assigning the connection you then have to specify type of STB so it was Sky TV and yours will then be Apple for V.Aux.
I couldn't use RGB as the Avant won't support it so bought an HDMI to Composite converter and sound and vision both come out of that into the Avant. I did try experiementing with using the optical output through a converter but the Avants card will only seem to work with the SAT and DVD sockets.
Hope this helps
Beovision Avant 32 RF, DVD1, Beovision 1, MX4002, Beound 3000, Beolab Penta MKII, Beovox Penta, Beolit 707, Beolink Passive, Beovox C30, Beocom 4, Beogram TX, 4 x Beo4, Form 1 & 2, Beocenter 7700, Beovox S65,