I've installed the tool unity-tweak-tool, but I couldn't start it from Dash.
I can launch it from terminal though. Please advise how can I make the dash icon work?
I assume you mean "Launcher".
This icon now should allow you to launch it from Launcher.
Please also make sure that you have properly installed it by following these steps:
sudo add-apt-repository ppa:freyja-dev/unity-tweak-tool-daily sudo apt-get update sudo apt-get install unity-tweak-tool