You access an IStoplists object by calling the ISearchSchema.Stoplists property, as in the following example, where objStoplists is the name you give to the IStoplists object:
Option Explicit
On Error Resume Next
Dim objSearchAdmin, objBuildServer, objCatalogs, objCatalog, objSchema, objStoplists
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
...
'Release objects
Set objStoplists = Nothing
Set objSchema = Nothing
Set objCatalog = Nothing
Set objCatalogs = Nothing
Set objBuildServer = Nothing
Set objSearchAdmin = Nothing