for the extension package is invalid error.
make sure to save the mxp file to your computer.
when you click the download link, you may have the option to open the extension, select the option to save it instead, once the download completes, open the extension manager and click the install buttn, browse to the mxp file to startthe installation.
for the other errors:
reset the workspace layout:
go to Window -> Work Space layout -> Reset '<your layout name>'
and Delete the Dreamweaver Cache file from the users configuration folder: