The alertmanagercore.resx file is used during installation. After the application is installed and running it will look for resources in the alertmanager.resx file. It looks for this file in your IIS site's APP_GlobalResources directory. Typically this directory can be found here...
C:\inetpub\wwwroot\wss\VirtualDirectories\{YourSite}\
The stsadm -o copyappbincontent command should copy the necessary resources into this directory but doesn't always work correctly. As a result you may have to copy this file over by hand. You can find the alertmanager.resx file in 12\Config\Resources\alertmanager.resx. Of course the 12 directory can be found at...
C:\Program Files\Common Files\microsoft shared\Web Server Extensions\12\