He means that you need to open Mysh River up, then reload packages again (separate from the package loading you have to do each time you start the program up). Those trees should be in the package called something like te-MyshkovaRiver, because every custom asset in a map has to saved in a separate package called by the name of the map, and if you save your custom assets to anything else or alter stuff and don't save it to that, it will screw with things. They are called Harley Pines.