Where is Address Book Server installed ?

Both the standard and the enterprise edition are installed in the /usr/share/... folder structure. As the services run in the background and are started automatically there is no need to perform any manual intervention.

Standard Edition Files and Folders

/usr/share/addressBookServer/
/Library/LaunchDaemons/com.addressBookServer.enterprise.plist

Enterprise Edition Files and Folders

/usr/share/addressBookServerEnterpriseEdition/
/Library/LaunchDaemons/com.addressBookServer.enterprise.plist

Neither edition can be accessed directly in the Finder, however it can be accessed via the "Go To Folder" option under the Go menu. Pressing Shift + Apple + G brings up the dialog, Enter /usr/share which will open up a Finder window from where you can access the rest of the files and folders.