using form validations with a modal window is going to take custom coding, the validation would need to be done through ajax. we can help you implement this in a premiere support appointment:
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.