when downloading the extension, make sure to select the option to Save it to your computer. You may have a choice to open it or save it, always choose the save option.
The error you are reporting usually happens if multiple versions of dreamweaver are installed and you double click the downloaded MXP file to start the installation.
Instead, start the installation manually after the extension is downloaded.
Open the Extension manager, then click the install button and select the MXP File you downloaded to begin the install.
for the error:
in dreamweaver go to Window -> Workspace -> Reset '<your layout name>' to reset the workspace, then Delete the Dreamweaver Cache file from the users configuration folder: