There isnt a checkbox in the buynow settings to make those fields required. If you want to make them required you can open up the page (step1.ascx and confirm.ascx) and add a required field validator control like this:
<asp:RequiredFieldValidator ID="rfvtxtUDTB1" runat="server" ControlToValidate="txtUDTB1" Display="Dynamic"
ErrorMessage="*Required" CssClass="SmithProdText"/>