· Microsoft SQL Server Native Client - sqlncli_x64.msi
· SQL Server 2005 Management Objects Collection - SQLServer2005_XMO_x64.msi
· Microsoft ADOMD.NET - SQLServer2005_ADOMD_x64.msi
These files all need to be installed on the Workflow Server host. I believe that each one is a dependency for the next one down in the list.