It's not working because you have assigned the onPress action of the sub buttons before the buttons are actually on the stage. So at the time you declare the onPress action the buttons do not yet exist.
Instead of setting the onPress actions in the root set them inside the itemGroup_mc in the frame in which they are physically on the timeline.
I got the forward button roll over working. by placing the code inside itemGroup_mc. I still cant get the button to actually play the frame inside that said clip.
I built another file before i made this one just to see how this all should work.
and it worked great. but when I built this one it didnt. and I built them the exact same way. as far as i know maybe you can look and see if you can see a difference.