We are creating a new iOS project. I want to use the Universal API so pretty sure I need to switch to Xamarin Beta channel and pull the prerelease of MVVMCross.
However when I try and add the MVVMCross package to a fresh PCL I get
Could not install package 'MvvmCross.PortableSupport 3.5.0-beta1'. You are trying to install this package into a project that targets 'portable-net45+sl50+Xamarin.iOS10+MonoAndroid10+MonoTouch10', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author.
The error looks simple enough but those compatibility errors from NuGet are never as simple as they look
The PCL is targetting
Which targets do I need to change so I can add the correct MVVMCross to build Universal iOS
It looks like Your challenge is supporting Silverlight 5 - sl50
All the other platforms you list will support modern PCL profiles like 78 and 259. However, Microsoft haven't built any Silverlight support for these profiles.
This dropping of the "older profiles" is why MvvmCross dropped the old WindowsPhone support - announced back in July 2013 http://slodge.blogspot.co.uk/2013/07/mvvmcross-wp7-tombstoned.html