Plenty of developers have not signed for their paid Apple Developer ID ($99/year) and signed their installers or standalone apps, resulting on GateKeeper not allowing to open them. Until they are code signed, follow this small tutorial to make them work. Plug-Ins do not need to be signed in order to work. Once the file has been downloaded. Click Dock Downloads folder. Click Open in Finder. Select the PKG installer by clicking it once. Click the tasks button. Click Open. Once the dialog box appear, click Open. The installer needs to copy files to your System folders like /Library/Audio/Plug-Ins/ and will ask for admin password for that. Follow installer steps.