When I try to update or to install dialog support package I get the error:
Could not install package 'MvvmCross.HotTuna.Touch.Dialog 3.5.0'. You are trying to install this package into a project that targets 'MonoTouch,Version=v1.0', 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 issue is you are still targeting the old 32 bit only iOS api (the MonoTouch.dll). All apps being written for the store must support 64 bit and 32 bit (Xamarin.ios.dll).
You can I believe get a build of mvvmcross 3.5 that will support the old apis, but I'd look at upgrading your project to support the new iOS apis. Xamarin produce some good docs here: http://developer.xamarin.com/guides/cross-platform/macios/32-and-64/.
This goes without saying but make sure you have everything in source control first before upgrading just in case. Once you have upgraded you should upgrade all mvvmcross components to 3.5