This is not like a mail script in an HTML site. The form information is processed by the plugin.
<form action='http://milindomexico.com/mlm/index.php?option=com_aclsfgpl&Itemid=1&ct=commun1&md=submitad' method='post' ENCTYPE='multipart/form-data' name='f'>
The plugin takes the information and writes it to the data base, it doesn't just put it into an email and send it to you like a simple form script.
There are ways for you to paste the dropdown into the page, but you would not be able to get any data because the classified ad program would not know what to do with it.
Look at the documentation for the classified program and see if it allows you to create extra fields. If it does not allow extra fields, the only thing you can do is modify the program. You would need a programmer to do it. You can't do it with Joomla or simple html.
You need to ask the developer if this is possible or hire a programmer to write the modification for you. It is not a simple task that can be handled by changing the template or design.
Everything in Joomla is database driven. If you are going to collect information from a form, there needs to be a place for it in the database.
I could be wrong, but I think this is the website of the company that makes that program
www.ads-programming.com/acj/ Try their website for help.
Cheers,
Ed