You access an IColumns object by calling the ISearchSchema.Columns property, as in the following example, where objColumns is the name you give to the IColumns object:
Option Explicit
On Error Resume Next
Dim objSearchAdmin, objBuildServer, objCatalogs, objCatalog, objSchema, objColumns
Set objSearchAdmin = CreateObject("Search.SearchAdmin.1")
Set objBuildServer = objSearchAdmin.BuildServer
Set objCatalogs = objBuildServer.BuildCatalogs
Set objCatalog = objCatalogs("KMSampleCatalog1")
Set objSchema = objCatalog.objSchema
Set objColumns = objSchema.objColumns
...
'Release objects
Set objColumns = Nothing
Set objSchema = Nothing
Set objCatalog = Nothing
Set objCatalogs = Nothing
Set objBuildServer = Nothing
Set objSearchAdmin = Nothing