The way I handle this in my editor is to
1) Create a copy of the object that is to be inserted and make it 60 to 70 percent transparent
2) Insert this object a set distance away from the camera (using the look/target vector)
3) Set all mouse movement to project the objects move in a single plane - The plane is determined by the camera's horizontal angle.. IE if the camera's target vector is greater than 45 degrees away from the xy plane vector, then the movement is in the xy plane.
4) After drawing the transparent version of the object, draw the object twice more using a stenciling mechanism to only draw the outline of the object - make this outline some solid color. On the second draw turn off depth checking so that the outline is always drawn.
5) Change the overall object color to transparent red if the user cannot insert the object at the current location.
In essence, you use whatever object you are trying to insert as an "object brush" that moves in a single plane following the users mouse movements. You can then set up a key (or a couple keys) to allow the user to change the plane of movement.
The outline of the "brush" will always be shown in the editor even when behind other stuff so the user is not confused, and you can change the color to red (or whatever color) to indicate that the object is not insert-able at its current location. When the user clicks in a valid location, you create an object in that spot and snap the object's position to whatever grid you want to use.
What kid of editor did you make? When I was writing the editor for my engine, I just used the standard 3 arrows in the XYZ and did the math for that. IMO thats the best solution, but I feel like that wouldn't be immersive enough to give to a player.