Add mesh circle, set the faces from ngon to triangles. Also make sure that you are NOT in wireframe. The circle should show in viewport + final render + viewport render. If it does not, please tell me! I would be happy to help further.
Here's a picture of setting this:
Here are pictures of the circle before and after:
Im Speculating that the orb wont just lay there on the plane, but maby gets some animation or interactive behavior.
The Blender file format is treated as an Prefab in Unity. Editing it further, adding scripts, components, rearranging, etc or unpacking it breaks connection and therefore the benefit of quickly updating edits from blender to unity vanishes.