Exchange 2007: Installation and Organization Prerequisites

I am installing the Exchange 2007 at Windows 2008 server. The Organization Prerequisites is FAILED. The error message: "The Active Directory Schema is not up-to-date and Ldifde.exe is not installed on this computer. You must install Ldifde.exe by running 'ServerManagerCmd -i RSAT -ADDS' or restart setup on a domain Controller.

The information is as the followings:
+1) I am logging in as a user called  “exch2k7_admin”
+2) The exchange server’s name is “EXCH2K7”
+3) Exch2k7_admin is a member of Domain User, Domain Admin, Enterprise Admin and Schema Admin; I also add this user as a member of Local administrator of EXCH2K7

The things that I have done to solve the above problem:
+1) I go to the exchange server called “EXCH2K7”
+2) I go to the command prompt following this path: start>all program>accessories>command prompt
+3) Right-click the command prompt> select “Run as administrator”
+4) I type in the command: “'ServerManagerCmd -i RSAT –ADDS”, but it failed

The error messages are as the followings:
ArgumentNotValid: Invalid role, role service, or feature: ‘-‘. The name was not found.

ArgumentNotValid: Invalid role, role service, or feature: ‘ADDS’. The name was not found

Any help?

tjie