Does the user click a button on the actual windows form or the website form before moving to the next item in the listbox?
If the user clicks a button on the winform, that's simple, use a button click event to move down the listbox to the next item, if there is no next item, move to the first one again (if that's what you want).
If it's a button on the website form they click to advance...I'm not sure, I never use the browser control.
See the ListBox.SelectedItem property and the ListBox.SelectedIndexChanged event.