Highlight the form field and go to Server Behaviors and click the plus button to choose Webassist->Security Assist->Show Region.
Then you will be able to show the field based on logged in status.
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.