Do you mean, the Media Browser is not showing the Aperture Library? What version of Aperture 3 do you have, and where is your Aperture library stored, on your system drive or an external drive?
For the Aperture Library to be visible in the Media Browser you need to have enabled the generation of previews in the Aperture "Previews" preferences. Do you have the option enabled "Share Previews with iLife and iWorks"? And have you the generation of previews enabled?
One other reason for Aperture not showing in the Media Browser could be a corrupted Aperture library. I suggest you try the "Aperture Library First Aid Tools" and repair your Aperture Library. Start with reapairing permissions, then try the other options, if necessary. See:
Another reason may be a corrupted preferences file; remove your Aperture preferences from your User Library, see Aperture 3: Troubleshooting Basics, the paragraph on removing preferences.