Add new namespace to project?

I created a new class to a project.
But the class is in a namespace:
namespace Project
{
//[Serializable()]
public class pagina
{
//
// TODO: Add constructor logic here
//
public int id;
Normally there is nothing wrong if i like to use this, i already used the same class (and namespace) in another project.
But right now he said that he could not find the namespace. if i in clude it like

using Project;

Whats wrong? And how can i use this namespace?
...