You access an IStoplists interface object by calling the ISearchSchema.Stoplists property, as in the following example, where objStoplists is the name you give to the IStoplists interface object:
Option Explicit
On Error Resume Next
Dim objSearchAdmin, objBuildCatalog, objStoplist
Set objSearchAdmin = CreateObject("Search.SearchAdmin.1")
Set objBuildCatalog = _
objSearchAdmin.BuildServer.BuildCatalogs("KMSampleCatalog1")
objBuildCatalog.Schema.Stoplists.Clear
Set objStoplist = objBuildCatalog.Schema.Stoplists.Add "Noise", _
"C:\objStoplists\Noise.enu", 9, 1
'Release objects
Set objStoplist = Nothing
Set objBuildCatalog = Nothing
Set objSearchAdmin = Nothing