your "please choose a store" should use a blank value instead:
<option value="">please choose a store</option>
then you can use the required / not blank / selection made validation type.
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.