The messagebox should be in the form not the class. The class should return a value, there are a few way of doing this, here's one:
//This is in the Class:
public class Class1
{
public string SomeMessage() {
return "Hello from Class1";
}
}
//This is in the form:
private void button1_Click(object sender, System.EventArgs e) {
Class1 c = new Class1();
MessageBox.Show( c.SomeMessage());
}