You set up BF2142 "vegitation" the same way you do that of FS.
1) Make new mod, call it bf2142 (semi-educated guess).
2) Copy your vegitation_server and vegitation_client zip files from the bf2142 directory to your desktop (or anywhere else, really) and extract them.
3) Put the extracted files into a folder called "vegitation".
4) Put this folder in the Objects folder of your newly created bf2142 mod folder.
5) Open up the editor, choose the "bf2142" mod, and load the vegitation using the Mod Manager. If "vegitation" doesn't show in the list as a checkbox, you've placed it in the wrong spot.
6) If your newly loaded trees don't show up in the LevelEditor, try restarting BF2 Editor.
"One of the bitches actually gave birth while she was attacking, and her puppies joined in on the carnage."
--the awesomeness that is Boatmurdered.