this error means that the download did not complete properly.
When you click the download link, your browser may prompt you to open the file or save it, do not select the option to open it, select the option to save it.
After you have downloaded the extension to your computer, try starting the installation process manually.
In Dreamweaver, go to Commands -> Manage Extensions to open the extesnion manager.
Once the extesnion manager is open, close Dreamweaver.
In the extesnion manager, click the install New extesnion icon. In the Finder Window, browse to the MXP File you downloaded and select it to begin the isntallation.