To use Callisto you must first have it installed. After installation then if using it in XAML markup you must declare the namespace. This page attempts to help you with both.

Callisto is available as a NuGet package that both installs it to your project and adds a reference to the Callisto APIs in your project. NuGet is per project so doing this in your project does not make it available in any new project's "add reference" view.

To use NuGet in your project file, simply right-click in VS and choose "Manage Package References" and search for Callisto, then choose Install.

If you wanted Callisto to be available on your dev machine for many projects and not use NuGet for per-project references, then you can install the Extension SDK. Installing it this way makes it available via "add new reference" dialog in Visual Studio to any project. Once you have it installed, you can right-click on your VS project, choose "Add Reference" and you will see Callisto in the Windows\Extensions area of that dialog. Select it and you are done.

Callisto is an API just like any other. When using in code, you could add "using Callisto.Controls" to your code file or however you like to use namespaces. This is no different than any other .NET API you would use.

If you are unfamiliar with using custom controls in XAML markup, you must first declare an XML namespace reference to the custom control. Let's say you have a Page you want to use Callisto in...you would first declare the namespace in the Page element:

<Page
	...
	xmlns:callisto="using:Callisto.Controls"
	...

Once you have this xmlns:callisto declared, then you can use it in your markup like:

...
<Grid>
	<callisto:Rating 
		ItemCount="5" 
		Value="3.7" />
...

And that's it!