Commented Unassigned: Cannot resolve Regions in XAML [10343]

I added http://www.codeplex.com/prism namespace to my XAML. It could not be found by intellisense but VS seemed to be happy with it manually added. When I try to add a region, I get an error "The name "RegionManager" does not exist in the namespace "http://www.codeplex.com/prism". I have not run into any issues with basic c# code, just in XAML.

I downloaded and am using Prism 4.1 against .NET 4.5 in VS 2012.

Is this a bug or am I missing something?

Relevant Code:
xmlns:prism="http://www.codeplex.com/prism"

prism:RegionManager.RegionName="{x:Static infran:RegionNames.AlertRegion}"
Comments: ** Comment from web user: LastDefense **

Is there something else that I need to reference besides those four dlls? I only downloaded those and service locator. Everything else I reference are regular .NET assemblies.