C# Connecting 2 Projects

C# Connecting 2 Projects

I downloaded a Sudoku game in C# and I am trying to connect that game with
my project but when I try to connect it with :
private void button4_Click(object sender, EventArgs e)
{
Sudoku.SudokuMainForm a = new Sudoku.SudokuMainForm();
a.Show();
Hide();
}
It tells me that Sudoku is a namespace but is used like a type.
Here is the part where I have problems with.
namespace Sudoku
{
public class SudokuMainForm : System.Windows.Forms.Form
}
Sudoku _newGame = new Sudoku();
private void btnAnswer_Click(object sender, System.EventArgs e)
{
_showAnswer = false;
ShowAnswer();
timer1.Enabled = true;
}
Tried the whole day to fix it,but can't find the solution for my problem.
Hope that you guys have an idea. Thanks