Site Import 1.x was not updated to be compatible with Dreamweaver CS4. You would need to upgrade SiteImport to SiteImport 3.
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.