Platform specific assembly references cannot be found

I have installed OxyPlot.XamarinIOS and OxyPlot.XamarinAndroid into their respective projects in a Visual Studio Xamarin Forms solution. As a result OxyPlot.Core is also installed and I can reference this perfectly fine. However the iOS and Android assemblies cannot be found with error messages:

using OxyPlot.XamarinIOS	//Cannot resolve symbol 'XamarinIOS'
using OxyPlot.XamarinAndroid	//Cannot resolve symbol 'XamarinAndroid'

I have checked that they match the OxyPlot.Core version, which is 2014.14.447. I have tried cleaning and rebuilding but nothing is working.