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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 change the version of . NET Framework to 4.5.
You are trying to install this package into a project that targets ' .Netframework,Version=v4.0'
See the removed text from the Xamarin.Forms requirements page
The PCL Solution template requires Profile 78 which targets . NET 4.5 - this version of . NET can be installed for Visual Studio 2012 (and is included in Visual Studio 2013)