Here's what I did and it worked. The input box returns an Excel range.
Excel.Workbook p_book
private void get_data()
{
Excel.Range Y_range;
Excel.Range X_range;
object missing = Type.Missing;
Y_range = p_book.Application.InputBox
("select Y range", "y range", missing, missing, missing,
missing, missing, 8);
X_range = p_book.Application.InputBox
("select X range", "x range", missing, missing, missing,
missing, missing, 8);
}