Commented Unassigned: ActionMessage Not in Namespace [358]

I am using VS2013 to create a WPF 45 project. Everything works fine except for the ActionMessage. I have tried setting up the namespace two ways:

1. xmlns:cal="http://www.caliburnproject.org"
2. xmlns:cal="clr-namespace:Caliburn.Micro;assembly=Caliburn.Micro"

Then, setting up a button to use an Action using either of 2 methods:

Method 1:
```
<Button Content="{Binding ConfirmButtonText}">
<i:Interaction.Triggers>
<i:EventTrigger EventName="Click">
<cal:ActionMessage MethodName="ConfirmCommand"></cal:ActionMessage>
</i:EventTrigger>
</i:Interaction.Triggers>
</Button>
```
Method 2:
```
<Button Content="{Binding ConfirmButtonText}"
cal:Message.Attach="[Event Click] = [Action ConfirmCommand()]" />
```

In all cases, the code runs as designed. However the page will not display properly in Blend or the VS designer because of an "Invalid Markup" error:

* The name "Message" does not exist in the namespace (whichever is referenced by cal:).
* The name "ActionMessage" does not exist in the namespace (whichever is referenced by cal:)

I want to use this product, but this is a showstopper. If the forms are not compatible with the designer or Blend, then I must abandon this otherwise awesome library.

Can anyone help?
Comments: I've seen the same issue as the reporter when using CM from a zip file and adding a reference. When I switched to the NuGet package the issue disappeared.