I opened "manage nuget packages for solution" - "browse" - and found what I was looking for. When I clicked on it I got "Installed: not installed". But I can't find how to install it. I already tried right clicking etc. - Nothing.
The graphical interface does not always work well. It seems that it has some problems and bugs! It's better to use nuget console. It's really easy and very fast. Just open it and type like bellow to install the package:
which X is the name of the package which you can find on the nuget website.
In addition, if you want to uninstall the package, just use 'uninstall' instead of install! The console also let's you do some other things like Migration, etc.