Validation on XML dropdownlist?

Hi,
im creating a registration form in ASP.NET (C#) .
in the COUNTRY dropdownbox i used datasource from my Country_db XML file. i can access all the country names perfectly.
but i dont know how to use Validator Tool on this XML's dropdownlist box. Validator Tools working perfectly on other normal textboxes.
i put an "Initial value" in requiredfieldvalidator but nothing happen.
Here is my coding in XML file:
 
<countries ValidationGroup="reg_form_validators"> <country Value="select" Selected="True" name="-select country-" /> <country Value="AF" name="Afghanistan" /> <country Value="AL" name="India" /> <country Value="DZ" name="UK" /> </countries>  
and here is the dropdown box code, where i want to access & validate it.:
< ...