To update the definition of the user class in the Active Directory schema with the MSS user attributes, you can import the LDIF file ms-user-plus-mto.ldf, which should have been included with this document. Typically, you'll use a command line like the following:
ldifde -i -f ms-user-plus-mto.ldf -k -v -j . -c "DC=X" #schemaNamingContext
(Specify this command all on one line.)
For those unfamiliar with the ldifde command, the parameter #schemaNamingContext is a symbolic constant that will be replaced with the DN of the schema in your LDAP directory. The parameter DC=X is a placeholder in the LDIF file that will be replaced with the value of #schemaNamingContext by ldifde as it processes the file. So where the LDIF commands in the file specify "DC=X", they will actually update the LDAP schema.
If you are using a class other than user for your Windows users, you'll have to modify this LDIF file before importing it. See your Active Directory documentation or other LDIF reference material for more information on LDIF files.