If you have created a scene that contains one or many of these assets (for example, a custom character for a game project, or a set of skins to be applied to a random character in a game) you may have noticed the following behavior: When you view the content of the scene and try to select the asset, it says it's ready, but no matter what you click on, it won't go to the scene. The same goes for editing the asset on the main menu window, reloading the game, or rebundling the asset to your project. This is likely due to a mistake in the sender's Unity project.
As Unity recompiles your scene every time you change it, it's possible an asset didn't get updated for whatever reason. If this is the case, in addition to saving the scene to the file, try saving it to the unity project, then the assets should show up in your scenes.
Until I create new Scenes in my game, I do all of my assets in the Unity project itself, as I make my level, and I have found it to be the easiest way to setup the asset. You can then open the Unity project, delete any new scenes, and drag all of your assets from the scene into the project. Then you can delete the scene files, since Unity will recreate them when you start a new scene. You can then drag the assets into any new scenes that you create.
Unzip the content of the.unity3D archive file you downloaded off of various sites. Drag all of the Scene files into your project. You may also have to drag the level.asset into unity. However, if you exported the whole level as a scene, you could drag the whole thing into unity. d2c66b5586