I am setting up a new system and downloaded Hazel 3.3.7 from your website today. After installing the Hazel preference pane, it displayed a warning dialog of "HazelHelper can't be opened because the identity of the developer cannot be confirmed".
This is on a new install of OS X 10.11 with the default security settings that only allows apps to run from the Mac App Store and from identified developers.
So, is there something wrong with your developer account/certificate or did you forget to sign when building? I was able to resolve the problem myself by removing the quarantine attribute on the pane but this is not something a typical user would know how to do.