Open the mission editor, press F4, look for a category called static shapes. Click the plus sign next to it, then look for another category called sharp trees, or something similar to that. Click the plus sign next to it, and then click a tree name to place it.
You can click and drag the tree to move it. If you hold control and click it, you can raise and lower it. You will also see three lines coming out of it. Click one of them (it's called a gizmo axis) and drag one. It will move only in the direction the line points in. If you hold down alt and click on an axis and move, you will rotate the object, which is currently the easiest way to rotate stuff. The other way requires you using the axisToEuler command to get the right rotation.
You can see more information on the axisToEuler command here:
http://www.blockland.us/smf/index.php?topic=27230.0Basically, you would input how much you wanted to rotate your object on each axis in degrees. The console would give you a different number. Now if you press F3 and select the object in the mission tree (it's that area on the upper right side of the screen filled with lots of blue text and plus signs) and look for something with the same number as your object (there should be a number on your tree that corresponds to one of the staticshapes in the mission tree), you should see a few options, one of which is called Transform. Click it, select the rotation field, and put that number in you got from the console. Then click apply.
