Thanks to the JDownloader Dev-Team admin, the JDFeedMe has been signed for JDownloader version 0.9.580 (the current stable version)!
You can get the signed version here. It should work now.
Don’t forget that JDFeedMe is currently actively developed, so there may be some bugs in the plugin. You are always welcome to report them and help the development team release a stable bug-free version.
If you want to be a beta tester, consider working with JDownloader Nightly (JDownloader beta), since the JDFeedMe for Nightly will be updated more frequently. Read about Nightly and how to install it in the JDownloader forum.