Object Editor
There are three major things that you can do with this tool.
Moving
On the vertical tool bar, if you select the tool that has arrows on its icon, you have selected this tool. You can use this tool in one of two ways. Number one, you can go into the scene tree and pick the "MissionGroup" icon file and then select the object you would like to move around. When you do this the object will light up and will have a blue arrow facing toward the sky, a green one facing to one side and a red one facing another way. The arrows should all meet creating a 90 degree angle. Then click on the arrows to slide the objects wherever you want. You may hear these axis being referred to as x,y,z but for a beginner I won't get into that as much. Basically, which ever line you move on, that is the only way it can move. If you select the y-axis (vertical one) you cannot move the object side to side, just up and down. This should be straight forward.
Rotating
On the vertical tool bar, if you select the tool that looks like a sphere with red, blue, and green lines, you will have this tool. Again, you can do this one of two ways. Now select the line that you want to move on. This takes a little bit of getting used to but in the end it's pretty easy. Now go ahead and put the object at whatever angle you want.
Scaling
On the vertical tool bar, if you select the tool that has boxes on the head, you have this tool. Now this can be a little tricky. The green box makes your object longer. The red box makes your object wider or fatter. And the blue box makes your object taller. On many models if you only change the position on one or two of the boxes it may not look correct.
For vegetation and trees
Instead of using the Object Editor to place trees and vegetation, use the Forest Editor as it is much more efficient.