AU (Audio Unit) plugins are .component files located in the user or system Library/Audio/Plug-ins/Components folder. OS X comes with several AU plugins, and many third parties offer AU plugins as well.
Virtually all AU plugins are AUv2 plugins. Some new ones are AUv3. MultitrackStudio supports both versions.
AU plugins can be loaded in an effect slot using the slot's down arrow. The AU plugins appear in the effect selector's "VST and AU plugins" section. The plugin will be shown in a window with Bypass and Presets buttons. Any presets coming with the plugin appear in the Presets menu.
If an AU plugin gets in a bad state somehow you can press Command-R while the plugin interface is visible. This will save the settings, reload the plugin and load the settings again.
Some plugins generate MIDI data. This MIDI output is merged with the data coming from MIDI input devices if the plugin is in a recording audio track and Soft Monitoring is enabled.
Note: if you install AU plugins while MultitrackStudio is running you may have to restart MultitrackStudio in order for the new plugin to be listed (the plugins are collected only once).