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