What version of Site Import are you using? What version of Dreamweaver? Also what is the serial number you are using (please post that in the private message area). I'll need more information to debug the cause.
Site Import was discontinued as a stand alone extension more than three years ago at this point. You may want to consider an upgrade to Design Extender in order to take advantage of new functionality and make sure it is supported on more recent Dreamweaver versions.