You want to use the Active Directory Schema snap-in for the first time on a computer.
Before you can use the Active Directory Schema snap-in, you have to register the dynamic link library (DLL) associated with it. This can be done with the regsvr32 utility using the following command:
> regsvr32 schmmgmt.dll
If the command is successful, you'll see the following message:
DllRegisterServer in schmmgmt.dll succeeded.
Most of the Active Directory MMC snap-ins do not require that you manually register the associated DLL. Microsoft requires this with the Active Directory Schema snap-in due to the sensitive nature of modifying the schema. This doesn't actually do much to prevent users from using it, but at least it isn't available by default. And regardless, only members of the Schema Admins group have permission to modify the schema anyway, so making this snap-in available should not pose much of a risk.
The schmmgmt.dll file is installed as part of adminpak.msi or when a domain controller is promoted. If you want to use the Schema snap-in on a non-domain controller machine and you have not installed the adminpak.msi package, you'll need to specify the full path to schmmgmt.dll when using regsvr32, which can be found in the \i386 directory of a Windows Server CD.
MS KB 320337 (HOW TO: Manage the Active Directory Schema in Windows 2000), and MS KB 326310 (HOW TO: Manage the Active Directory Schema in Windows Server 2003 Enterprise Edition)