You create an IStoplist object by calling the IStoplists.Add method, as in the following example, where objStoplist is the name you give to the IStoplist object:
Option Explicit
On Error Resume Next
Dim objSearchAdmin, objBuildServer, objCatalogs, objCatalog, objSchema, objStoplists, objStoplist
Set objSearchAdmin = CreateObject("Search.SearchAdmin.1")
Set objBuildServer = objSearchAdmin.BuildServer
Set objCatalogs = objBuildServer.BuildCatalogs
Set objCatalog = objCatalogs("KMSampleCatalog1")
Set objSchema = objCatalog.objSchema
Set objStoplists = objSchema.objStoplists
Set objStoplist = objStoplists.Add("C:\objStoplists\Noise.enu")
...
'Release objects
Set objStoplist = Nothing
Set objStoplists = Nothing
Set objSchema = Nothing
Set objCatalog = Nothing
Set objCatalogs = Nothing
Set objBuildServer = Nothing
Set objSearchAdmin = Nothing