When i put the ODF's of a level and i find a tree, most of the times it hasn't any texture, and any leaf. Can someone tell me how can i add some trees?

you need to add the .msh folder from the same folder you found the .odfs in. *cough* *cough* read the documentation

Trees appear slightly transparent with no leafs in ZE (naboo tree for example) but ingame they are fine.

ok, thanks penguin. And i've read the tutorial.
But thanks to you yankfan too.