The OS X version of Lightroom 2 and older have a bug with rendering panels that causes interior content to sometimes overextend the width of the column. It doesn't happen all the time, but it will happen from time to time.
To fix the issue, hover over the vertical divider between the work area and the panels, click, hold, and expand the panels all the way to the left until you stop. Then quit/relaunch Lightroom. The panel content should reset and be back in view.