In Plan view mode choose Insert furniture button. Then select Custom furniture. Push Add new button and you get window like 1st picture.
Select Custom object tab and then Browse and enter the path of your new 3ds-file. You can choose from Subdirectory drop down menu the category where to put the new object.
Next go to the Size tab and checkmark in Scale-area Custom and enter value 10.
In Material tab you can modify the materials and colors of the image.
Last go to the General tab and rename (if you want) the object. After clicking OK, you are ready to use your new object.
Insert objects to plan... even inside of other object
Sometimes, when you have many objects in same view, moving a new object to a exact location is almost impossible.
There is a very easy way to handle this. Click to activate the object you are going to move so it turns green. Then click Display filter in the toolbar. Select Hide all an then OK. Now your object is the only one in sight, and moving it is quite easy. You may have to move the object few times or make a mark to your monitor ;).
Using invisible textures in FP3D v.5
You need Microsoft Access to do this. Take a backup copy of file Flrplan.mbd, then open it in Access. Open table Property. Click OK if needed (I had to 3 times). Then add New data record 
(I dont know if it is new data record in English, cause my Access is in Finnish, but the button is like in picture). You will get a new row. Write Clear texture to cell Property Name, write 100 to cell Trasparency, all other cells gets value 0. Last three columns can be _COLOR = PLAIN, _REFLECTANCE = MATTE, _TRANSPARENCY = PLAIN. I got these values and it works.
Now you can define texture to clear, so it will come completly invisible. Benefits are, for example, you can create your own lamps. Define existing lamp's texture to clear, put your own lamp in place of it and that's all.
Fire in the fireplace
This fireplace's fire was created with recessed light 35 watts, orange filter. The fireplace is empty inside. I created a new location which floor level was 80 cm higher than floor level I put the fireplace. Then I draw 30 cm x 30 cm, 1 cm high invisible walls (this is important, because without the walls, lamp is not going to stay where it is, after editing) and inserted lamp to this rectangular. If you have trouble inserting lamp to plan, look tip Insert objects to plan