I fixed the file download issue it was an ajax request issue.
Now the extension install issue.
It should install I only tested on OSX with DW 8.0
but should install Peter.
This issue is most likely your extension installer for DW you need to update it.
Get it here
Macromedia - Exchange : Download the Macromedia Extension Manager
This should fix any issues that your having on install.
Cheers,
Brian