DataAssist 1.3.1 is not supported in Dreamweaver CS3. it was released roughly 2 months before CS3 was.
To use DataAssist in Dreamweaver CS3 will require upgrading to DataAssist 2.
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.