Xamarin.Forms.Core could not be resolved

just getting started with XS and Xamarin Forms. Downloaded all the sample apps and now I'm getting this compile error:

C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(5,5): Warning MSB3274: The primary reference "Xamarin.Forms.Core" could not be resolved because it was built against the ".NETPortable,Version=v4.5,Profile=Profile259" framework. This is a higher version than the currently targeted framework ".NETPortable,Version=v4.0,Profile=Profile131". (MSB3274) (WorkingWithMaps)

Do I need to install something else to get profile259?

thanks!
brian