One question for someone?!
 

I'm just a beginner with Maya. I have problem with rendering. When I put tree from Visor window and render with Maya software render tree appears but when I render it with Mental ray there is no tree - how can I solve this problem?!

Paint effects done render with mental ray, so unless you convert it to polygons you cant.

How do I do that? :-))

Sorry but I'm very new in it

Select the tree, then under 'edit' (think thats the right menu) got to 'modify' > 'convert paint effects to polygons'.

Chris