1. Unzip the files
2. go into World editor.exe
3. press F12
4. press CTRL + I and loop yourself until everything is imported, "OR" CTRL + SHIFT + I to import everything in a folder.
5. Save your map
6. go to destructible editor, and check the field below the original models,
7. there must be "warcraft3imported/trees.mdx" files, thats the models you imported.
(In your case, you don't need any texture importing, the trees uses in-game textures.) so i skipped texture import part.