Hi Nathanael,
Could you tell me why you want to install the plugins manually in the first place? Does your SCM-Manager not have access to the plugin server, or is there another reason? I'm asking, because the manual installation can always lead to issues. Therefore the automatic installation, by using the "install" button in the plugins screen is the recommended way.
Beyond that, I would like to know where you got the zip file. Did you download the repository from Bitbucket? If that is the case, then the zip file does not contain executable code.
Cheers,
Daniel