i have the freely downloaded full version of unity on my windows 7 based pc.
First off i cant see the grid in scene view its just blank space.
Also i cant see any 3D models put into the scene neither can i see the wireframe for any model i put in,
1st screenshot is what i see when i load up the unity demo files.
2nd screenshot is one of my exported fbx 3D models made in 3DS max but the wireframe you can see will disappear when i manipulate it and the object will be gone all together. cannot select it.
Chances are you clicked the layers button, near the top left of the editor, and unchecked default by accident. This will pretty much disable you from seeing any objects you are adding to the scene view. That would likely explain the reason why you were not seeing the models you were adding.