how can i attack trees?

how can i make my unit attack a doodad tree?

Go into the valid attack targets for the unit and click on "Trees."

ok i did that but it dont work

i got a regular hero and changed the model to the brew master

would that cause the problem?

Shouldn't.

Take a look at the valid targets for the Undead Goul. That should help.

Is it a 'doodad' tree, or is it 'destructible' tree..? The 'doodads' cannot be attacked by the 'targets allowed: tree'.. ''. If you get my meaning.

It is most likely 'destructables.'

However, Arthas can attack a crate or barrel, but no a tree.

a barrel is not targetred as a tree ;)
to attack trees you have to order the unit, by klick the attack order and then select the tree you want attack(after you made trees as a possible target for the unit)