You access an ISearchCatalogs object by calling the ISearchServer.SearchCatalogs property, as in the following example, where objCatalogs is the name you give to the ISearchCatalogs object:
Option Explicit
On Error Resume Next
Dim objSearchAdmin, objSearchServer, objCatalogs
Set objSearchAdmin = CreateObject("Search.SearchAdmin.1")
Set objSearchServer = objSearchAdmin.SearchServer
Set objCatalogs = objSearchServer.SearchCatalogs
...
'Release objects
Set objCatalogs = Nothing
Set objSearchServer = Nothing
Set objSearchAdmin = Nothing