Well, the plug-in sentinel finds Kontakt5 in /Library/Audio/Plug-Ins/VST/ but for whatever reason, it detects it to be problematic and blacklists it, so no wonder you will not be able to select it from within Dorico.
Could you tell me the exact version number of your Kontakt5 installation? So far I’m not aware that a specific version of Kontakt will get blacklisted, but I wouldn’t rule anything out.
I have a Kontak 5.5.2.0 installation here and that loads fine into Dorico.