Quick question regarding the level editor

That's an issue strictly linked to what's the content in 'your' (or the level you are trying to mod) folder.
If it's an official map, that content is more likely in that folder already. Try going in the 'forest editor' and you should see them.

 

Nah, Your texture are missing because the paths in the material.cs are different problably.
Just find the material.cs in the folder you copied over, open in notepad and adjust the paths.

 

To add forest items to a level you need to do the following:

1. Copy the files over, such as "levels/east_coast_usa/art/shapes/trees/trees_aspen"
2. Edit the material file to point to your new level, it might say ""levels/east_coast_usa/art/shapes/trees/trees_aspen_d.dds"", you need to change east_coast_usa to your map folder
3. Add the appropriate entries to the "levels/your_map_name/art/forest managedItemData.cs (this defines what actual meshes are used by the system and what they're called) and brushes.cs (this tells the game how to list the items and how to place them) - the easiest way to do this is open the two files from the level you're copying from as well as the two you have placed in your own level file and copy each item from the old to the new.