Accessing the plugins and macros folders
To access the plugins or macros folders, set the Finder window to either icons or lists mode, not in column mode, and double-click them.
Alternatively, right-click (or control-click) the Fiji.app and select "Show package contents", to open the folder where the actual plugins and macros folders are.
Adding new plugins and macros
Access the plugins folder as explained above and just drag and drop any plugin into the plugins folder, like you would do for ImageJ. Same for macros.