Multiple projects in one solution

Question

Hi there,
I created a solution with multiple projects (for a game, different levels).
I have a start screen, and when you click the "Start button" i want them to open another solution, how can i do something like that?

I've tried this:
 
MiniGame_PressRedButton.Game1 game1 = new MiniGame_PressRedButton.Game1();
game1.Show();
 
But that results in this error: Error1The type or namespace name 'MiniGame_PressRedButton' could not be found (are you missing a using directive or an assembly reference?)
Thanks