You create an ISitePath object by calling the ISitePaths.Add method, as in the following example, where objPath is the name you give to the ISitePath object:
Option Explicit 
On Error Resume Next
Dim objSearchAdmin, objBuildServer, objCatalogs, objCatalog, objSites, objSite, objPaths 
Set objSearchAdmin = CreateObject("Search.SearchAdmin.1")
Set objBuildServer = objSearchAdmin.BuildServer
Set objCatalogs = objBuildServer.BuildCatalogs 
Set objCatalog  = objCatalogs("KMSampleCatalog2")
Set objSites    = objCatalog.objSites
Set objSite     = objSites.Item("Microsoft.com")
Set objPaths    = objSite.objPaths
Set Path = objPaths.Add "C:\Tmp", False 
...
'Release objects
Set Path     = Nothing 
Set objPaths    = Nothing 
Set objSite     = Nothing 
Set objSites    = Nothing 
Set objCatalog  = Nothing 
Set objCatalogs = Nothing 
Set objBuildServer = Nothing 
Set objSearchAdmin = Nothing