You access an IMappings interface object by calling the IBuildCatalog.Mappings property, as in the following example, where objMappings is the name you give to the IMappings interface object:
Option Explicit
On Error Resume Next
Dim objSearchAdmin, objBuildCatalog, objMappings
Set objSearchAdmin = CreateObject("Search.SearchAdmin.1")
Set objBuildCatalog = _
objSearchAdmin.BuildServer.BuildCatalogs("KMSampleCatalog1")
objBuildCatalog.Mappings.Add "C:\InetPub\wwwroot", _
"http://www.microsoft.com"
' See Creating and Accessing IMapping Interface Objects
'Release objects
Set objMapping = Nothing
Set objBuildCatalog = Nothing
Set objSearchAdmin = Nothing