JDownloader 2 is not in a stable final state. The current JDownloader 2 version is meant to be used for testing purposes only. This means that there will be unfinished features, bugs, many updates and even untested modules, code & plugins.
These installers require a internet connection for installation, because they download the latest version directly from our update servers.
Don't say you haven't been warned!
FOR YOUR INFORMATION (15 Aug 2014)
Due to changes of our codesign certificate it might happen that you get some warnings during download or execution of the install tool.
You might have to access the downloaded file in the download list of Google Chrome.
You will have to ignore the warning about missing reputation until Microsoft collected enough statistics about the installer.