Matrices using text box

The easiest way is to create a constructor for the second form that takes an integer value (or two integers - x and y for future expansion) and uses that to construct the appropriate data for the user to fill in. You also create a Property in the matrix form which returns the result. You then read the value in the first form, convert it from a string to an int, and construct the new form instance with the number:
int size;

if (!int.TryParse(myTextBox.Text, out size))
   {
   // Report problem to user - it's not a number!
   }
else
   {
   frmMatrix fm = new frmMatrix(size, size);
   fm.ShowDialog();
   myResultTextBox.Text = fm.Result.ToString();
   }

Exactly how to display it in the matrix form is up to you - there are a lot of possible options you could use. Any idea what you would be comfortable with?