You should be looking at the "#Partition_Left_Wrapper" id in the CSS code. It's the third one down.
I am following a tutorial, but the guy does not use a external stylesheet. Not sure if this affects css3 syntax/layout (relatively new here), but I can't seem to access the child block from the parent:
#parent_DIV_Wrapper:hover child_DIV {display:block;}
How exactly do I lay out the wrapper and child div to get the hide/display effect I am after?