When you get the error “DllNotFoundException: libHARC.dylib” (or something similar) when you try to open one of the menu items under “Houdini Engine” menu tab you should open up Console.app. Not Unity's but of the OS. Just use spotlight search. In the sidebar find the Unity folder should be under “~/Library/Logs”. Open up the Editor.log file and scroll down it should say something like “Fallback handler could not load library /Applications/Unity/Unity.app/Contents/Frameworks/Mono/lib/libHARC.dylib”
The simplest solution is to just create a symlink in that folder to the library. Open up terminal and add these commands (Ofcourse change the location if you have Houdini or Unity installed in a different location and change the dylib filename if you have a different error)
ln -s /Applications/Houdini/Current/Frameworks/Houdini.framework/Versions/Current/Libraries/libHARC.dylib /Applications/Unity/Unity.app/Contents/Frameworks/Mono/lib/libHARC.dylib
There might be a better solution like adding a DLLmap to the mono config file or adding environment variables to DYLD_FRAMEWORK_PATH or DYLD_LIBRARY_PATH (both of these options didn't work for me btw) but this a much easier solution in my opinion.