I have the exact same problem as NN2RL. Searching the web brings the same fix that Frank points out that did not work for me or NN2RL:
"The media browser shows the last library opened by iPhoto or Aperture. If you want a different library viewable in the media browser for Aperture you will have to have that library be the last opened library in Aperture."
However, I did some experimenting by creating some new aperture libraries and what I find my MAC OSX Mavericks is doing is showing the LAST CREATED aperture library regardless of the last one opened or default aperture library.
Anyone have this problem and solve it? There must be some setting somewhere that can update the aperture library used in the media browser.