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