Its not possible to change chat servers using yahoo messenger(american version)only american chat servers(english servers)are pre programmed into the yahoo messenger american version.You will have to use yahoo messenger(mexican version)or argentinian version if you want to use foreign servers,
Or use yahelite or yazak because they are third party clients and are not contracted to yahoo so they are programmed with all servers included.
I answered you question in another topic, use the /join (roomname) command if you want to get to the foreign rooms using yahoo messenger(american version)because there is no program you can add to yahoo american version that will allow you change to international servers.