From Jesse Liberty's page on Creating Controls in Code:
Button button2 = new Button();
button2.Width = 75;
button2.Height = 30;
button2.Content = "No, click me!";
button2.SetValue(Grid.RowProperty, 0);
button2.SetValue(Grid.ColumnProperty, 1);
LayoutRoot.Children.Add(button2);
You'd do the same thing with your own user control. Just replace "new Button()" with the name of your user control, e.g. "new MyUserControl()".
So a simplified version may be something like this:
AwesomeControl _AwesomeControl = new AwesomeControl();
LayoutRoot.Children.Add(_AwesomeControl);