You access an ISearchSchema object by calling the IBuildCatalog.Schema property, as in the following example, where objSchema is the name you give to the ISearchSchema object:
Option Explicit
On Error Resume Next
Dim objSearchAdmin, objBuildServer, objCatalogs, objCatalog, objSchema
Set objSearchAdmin = CreateObject("Search.SearchAdmin.1")
Set objBuildServer = objSearchAdmin.BuildServer
Set objCatalogs = objBuildServer.BuildCatalogs
For Each objCatalog in objCatalogs
Set objSchema = objCatalog.objSchema
...
Next
'Release objects
Set objSchema = Nothing
Set objCatalog = Nothing
Set objCatalogs = Nothing
Set objBuildServer = Nothing
Set objSearchAdmin = Nothing