1. Create any model in FreeCAD
2. In FreeCAD select File->Export and chose a destination, name, and save it as .obj.
3. Run command prompt in the directory where you saved your .obj file.
3. Convert your .obj model to .g model by typing for example following command (there a lot of ways to convert .obj file to .g file) in your command prompt:
   obj-g -u mm -i model_name.obj model_name.g
4. Open .g file as database in MGED. Type "tops" in command window, and draw object by typing: draw object_name.