XNA probably suppresses keyboard input. This took me a while to solve but to go about this use event KeyPreview and inside set e.IsInputHandled to true. or use this code as your textbox class:
public class RespondingTextBox : System.Windows.Forms.TextBox
{
public RespondingTextBox()
{
PreviewKeyDown += txtBox_PreviewKeyDown;
}
void txtBox_PreviewKeyDown(object sender, System.Windows.Forms.PreviewKeyDownEventArgs e)
{
e.IsInputKey = true;
}
}