Can I import models from Lego Digital Designer to Unity and how ?
No you can’t. The LDD uses a zipped xml format that only stores what bricks you’ve used. There is no mesh information at all when you export it. Actually I’m pretty sure the brick meshes are copyrighted by LEGO, so you aren’t allowed to use them elsewhere, even when the export would be possible.
you can if you use leoCAD and then convert it to blender or converter your LDD files to 3dsMax and then put your converted 3dsMax and or Blender file to unity.
there is only one problem well at least with mine, it doesn’t render properly in unity, but in Blender it renders fine.