You create an IStartPage object by calling the IStartPages.Add method, as in the following example, where objStartPage is the name you give to the IStartPage object:
Option Explicit
On Error Resume Next
Dim objSearchAdmin, objBuildServer, objCatalogs, objCatalog, objStartPages, objStartPage
Set objSearchAdmin = CreateObject("Search.SearchAdmin.1")
Set objBuildServer = objSearchAdmin.BuildServer
Set objCatalogs = objBuildServer.BuildCatalogs
Set objCatalog = objCatalogs("KMSampleCatalog2")
Set objStartPages = objCatalog.objStartPages
Set objStartPage = objStartPages.Add("http://www.microsoft.com")
...
'Release objects
Set objStartPage = Nothing
Set objStartPages = Nothing
Set objCatalog = Nothing
Set objCatalogs = Nothing
Set objBuildServer = Nothing
Set objSearchAdmin = Nothing