Arranging objects in a scene can be easy but frustrating. The easy part is that there are many ways to arrange objects, including dragging them with a mouse or typing in your own coordinates. The frustrating part is that sometimes your objects don't align exactly.
Right now you have three parts of a mail box scattered in the scene and no clue what to
do with them. First, we'll line up the mailbox with the base. To do this click
the mailbox with the selection tool 
. Then, click the box on the back plane that
represents the mailbox and drag it directly over to the box that represents the mail
box post. If you use this method (works better) you will also have to move the
corresponding box of the mail box main part to the box representing the post on one of the
other grids left. Look below to see what I mean.
Actual Screen may Vary
Now that the main mail box part is aligned with the post, you will have to stretch the object three squares toward you by selecting the object and dragging one of the small squares on front of the box on the right grid three squares toward you. The object should look like this.
After doing this you might want to thin the object by clicking one of the square on the left grid and dragging it toward the center. After thinning, you will need to duplicate the mailbox again because the original duplicate will not fit the mailbox correctly. Delete the original duplication after duplicating the object again.
After duplicating the object, move it somewhere and thin it using the same
method of enlarging the mailbox in reverse. Thin this part to about a one box or
half a box. Arrange it until the object you just thinned is in front of the mailbox
and all grid boxed that represent the objects are touching. Now, using the rotation
tool, 
select the mailbox cover (the thinn object). Click and drag the the cover down until
it looks like the mail box is open. After doing this align the cover again.
Now the object is completely put together and is ready to be painted. Your object should
look something like this.
