You access a specific ISearchCatalog object by calling the ISearchCatalogs.Item property. Since ISearchCatalogs is a collection, you can access all ISearchCatalog objects using a for each loop, as in the following example, where objCatalog is the name you give to the ISearchCatalog object:
Option Explicit
On Error Resume Next
Dim objSearchAdmin, objSearchServer, objCatalogs, objCatalog
Set objSearchAdmin = CreateObject("Search.SearchAdmin.1")
Set objSearchServer = objSearchAdmin.SearchServer
Set objCatalogs = objSearchServer.SearchCatalogs
For Each objCatalog in objCatalogs
...
Next
'Release objects
Set objCatalog = Nothing
Set objCatalogs = Nothing
Set objSearchServer = Nothing
Set objSearchAdmin = Nothing