You access an IExtensions object by calling the IBuildCatalog.Extensions property, as in the following example, where objExtensions is the name you give to the IExtensions object:
Option Explicit
On Error Resume Next
Dim objSearchAdmin, objBuildServer, objCatalogs, objCatalog, objExtensions
Set objSearchAdmin = CreateObject("Search.SearchAdmin.1")
Set objBuildServer = objSearchAdmin.BuildServer
Set objCatalogs = objBuildServer.BuildCatalogs
Set objCatalog = objCatalogs("KMSampleCatalog2")
Set objExtensions = objCatalog.objExtensions("KMSampleCatalog1")
...
'Release objects
Set objExtensions = Nothing
Set objCatalog = Nothing
Set objCatalogs = Nothing
Set objBuildServer = Nothing
Set objSearchAdmin = Nothing