Problem compiling pages derived from ExtendedTabbedPage

Hi,

I am trying to use the ExtendedTabbedPage in a Xamarin.Forms shared code project. When I compile I get the following error:
error CS0246: The type or namespace name 'ExtendedTabbedPage' could not be found (are you missing a using directive or an assembly reference?)

Do you have any idea? We have another developer on the project where it works fine. The other developer created the page, however, also when I create a new xaml page and try to derive it from ExtendedTabbedPage I have the same problem.

Any thoughts on that?

Thanks