I find the built in geotagging features of Aperture maddening — it is just way to hard to find a location on a map and assign it to the current photo. So I wrote a simple apple script that assigns the current location in google earth to any selected photos in aperture, which you can use at your own risk.
Download Geotag With Google Eearth.scptd to /Users/doug/Library/Scripts/Applications/Aperture, replacing “doug” with your username. You will probably have to create the Aperture folder under /Users/…/Library/Scripts/Applications/. Then all you have to do is open Google Earth, navigate to the location of the image you wish to geotag. Then in aperture, select the images you want to geotag, and select the “Geotag With Google Earth” from the Applescript menu.