are you editing the current one, or adding a new one, it sounds like you are editing instead of adding a new one.
to add a new populate list from array behavior go to insert -> WebAssist -> Dynamic DropDowns -> Create dynamic Array
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.