Site import does not support Dreamweaver 8.
the System requirements for Site Import 3 are:
Windows
* Windows 7, Windows Vista, Windows XP SP2, or Windows Server 2003
* 128 MB RAM (256 MB recommended)
* Dreamweaver CS5, Dreamweaver CS4, Dreamweaver CS3
* Works with static, dynamic or template pages.
Macintosh
* Mac PowerPC or Mac Intel
* MacOS 10.4.x, MacOS 10.5.x, MacOS 10.6.x
* Dreamweaver CS5, Dreamweaver CS4, Dreamweaver CS3
* Works with static, dynamic or template pages.
you will need to upgrade your version of Dreamweaver.