Unity 4 Problem - GUI Skin can not be founded

I have a var of GUISkin type. In Inspector, I need to choose a GUISkin Asset for this variable. But when I click to choose, can not find my GUISkins.

I need this. I'm having this problem with Unity 4. (I converted my project from 3.4 to 4).

I am very upset about it. The update should improve...