The error message says it all: you are trying to install the package in a project with .NET 4.0, but the package is not compatible with it, if it is possible to switch the .NET Framework version to 4.5.
You are trying to install this package into a project that targets '.NETFramework, Version = v4.0'
See the Xamarin.Forms
The PCL solution template requires that Visual Studio 2013 is not supported.