In order to make the whole background a bit more clear the schema extensions are explained in detail here, so that in case of problems they can be checked.
In order to be able to see the details of the AD schema, ADSI Edit has to be installed on the server. This tool is available as soon as the Support Tools from the Microsoft Server CD have been installed.
To be found on the CD under: CD_ROOT/SUPPORT/TOOLS/SUPTOOLS.msi
Then the PlugIn ADSI Edit can be opened in the Microsoft Management Console (MMC). Now different connections can be established.