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