David,
First of all, make sure your library has no problems before you try to move it.
Run the Aperture Library First Aid tools to repair it:
See: Repairing and Rebuilding Your Aperture Library
Once the NAS is mounted on your iMac and the iMac sees it as a disk, simply use the Finder to drag the Aperture library from the NAS to the folder on your iMAC where you want to install it.
Then double click the Aperture library in its new location to open it in Aperture.
You will have to make sure that the NAS is mounted before you launch Aperture.
For more details see this section in the Aperture User Manual:
Working with Library Files
Should Aperture report missing original files after moving the Library, use the "Locate Referenced Files" dialog to reconnect the Originals.
See: Reconnecting Missing or Offline Referenced Images
Are you sure, you want to leave your referenced images on the NAS? There have been quite a few posts recently reporting problems with referenced images on a NAS.
Regards
Léonie