Site Import 1 is not compatible with Dreamweaver CS4.
to use Site Import in Dreamweaver CS4, you will need to upgrade to Site Import 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.