I'm using KDE now. In Unity, I could open Dash with Meta. I want to do the same in KDE - open Application Activator with Meta key, but I don't know, how. In this widget's properties I can't set it.
I don't have the Unity but this is of the Unity 2d: Is there a way to launch Unity Dash outside of Unity?
You could add those commands to the KDE Menu and assign a keyboard shortcut.
KDE Menu editor online handbook: http://docs.kde.org/development/en/kde-workspace/kmenuedit/index.html
Adding things to the K-Menu: Chapter 2. Adding a Menu Entry.
The advanced tab has the current shortcut key: