You will need to tell selenium to change framesI think, to do this you will have to use the selectFrameCommand:
selectFrame ( locator ) Selects a frame within the current window. (You may invoke this command multiple times to select nested frames.) To select the parent frame, use "relative=parent" as a locator; to select the top frame, use "relative=top". You can also select a frame by its 0-based index number; select the first frame with "index=0", or the third frame with "index=2". You may also use a DOM expression to identify the frame you want directly, like this: dom=frames["main"].frames["subframe"]
Arguments:
locator - an element locator identifying a frame or iframe
Once you have changed to the correct frame the mouse scroll should work