Go to the control in your Form where you want to set the data and change the "Modifiers" to internal or public (from privet). To do that go to the Form desing view select the control and on the control properties find "Modifiers". There the default value is privat. Just change it to internal or public.
For example if your control has the name txtTest you can do this:
MyForm frm=new MyForm();
frm.Show();
frm.txtTest.Text="Hello Worl";
This works only if txtTest is public or internal!!!