You access an IStartPages object by calling the IBuildCatalog.StartPages property, as in the following example, where objStartPages is the name you give to the IStartPages object:
Option Explicit
On Error Resume Next
Dim objSearchAdmin, objBuildServer, objCatalogs, objCatalog, objStartPages
Set objSearchAdmin = CreateObject("Search.SearchAdmin.1")
Set objBuildServer = objSearchAdmin.BuildServer
Set objCatalogs = objBuildServer.BuildCatalogs
Set objCatalog = objCatalogs("KMSampleCatalog1")
Set objStartPages = objCatalog.objStartPages
...
'Release objects
Set objStartPages = Nothing
Set objCatalog = Nothing
Set objCatalogs = Nothing
Set objBuildServer = Nothing
Set objSearchAdmin = Nothing