P3d models can't directly be placed in a mission, because they aren't completed objects. There has to be a config file attatched to them, in the form of an addon, before they can be used as an object in-game. For example, I could use that Dskrin_trojdvera.p3d model, and give it the config of a soldier, and I would end up with a playable-cabinet that can glide around and shoot at people ;D.
However, there are a bunch of objects that you can use by default, without any addons. Just place a unit, side=empty, and look under class "objects". There you can find lots of objects to place. But I'm sure you know that already.
There are also a number of objects that are not displayed in the editor, but are still found in the game. Houses are a good example. To place these objects, you need to place an object (side = empty), such as a sandbag wall. Next, save the mission and open up its mission.sqm file. Now scroll down until you find the object you just placed, and find the line where it says "vehicle = XXX". "XXX" is the name the game uses for the object you just placed, and can be different from what is displayed. If you change the text here with the name of the object you want, such as the house name, etc, then save the mission.sqm and re-load the mission in the editor, you can place all sorts of objects.
But how do you know the name of an object? Here's an easy method. First, find an object on the map and find it's ID. Now, place this line in an init field of a unit:
hint format ["%1",typeof object XXXXXX]
Just replace the X's with the ID of the object, and it will display the name of the object. Write this down and edit the mission.sqm file, as I described above, and you'll be placing houses, cannons, roads, and others in no time!
Note: for some reason, bushes and trees don't have a name, and so you can't place them using this method.