Update June 2013
Xamarin has now released their first PCL support.
Further, one simple workaround for this issue has been found: simply don't use MvxCommand
but instead use your own local-project MyCommand
implementation of System.Windows.Input.ICommand
as part of your Core
project.
End of Update
Xamarin doesn't yet support PCLs properly
As a result you must compile your pcl projects separately for ios builds - and you must reference dll's built for iOS rather than dll's built for 'proper PCL'
When following the current mvx walkthrough this just means that you need to use different mvvmcross binaries on the Mac. I generally do this by just putting different dll files in /lib/mvx folder
The repo https://github.com/slodge/mvvmcross-binaries contains four sets of binaries currently - these cover all of pcl and ios, debug and release
More on this situation is available in http://slodge.blogspot.co.uk/2013/01/almost-portable-binaries.html?m=1
Hopefully this situation is changing soon - as soon as proper PCL support arrives then we should be able to use the same set of binaries on all platforms :)