"clouds" are a somewhat irregular pattern of splotches across the range of colours in your foreground and background selections. Very often it is worth it to only render b&w clouds because it makes tinting them easier later.
In CS2 go up to Filter>Render>Clouds. It will fill the current layer. Its far from perfect. The clouds tend to have regularities to them. Very big canvasses often produce clouds with a whole bunch of little puffs rather than large ones. The trick in this case is to fill a smaller layer and stretch it across what you want to cover.
Hope that helps.