To initialize MBMessages you need to add MBMessages to the array of plugins of MBurger.

val plugins = ArrayList<MBPlugin>()
val plugin = MBAudience()
MBurger.initialize(applicationContext, "MBURGER_KEY", false, plugins)

Then you need to say MBurger to initialize the plugins with the in initPlugins method from MBurger class. If you wish you can add a listener to know when the plugin have been initialized.

val listener: MBAudiencePluginInitialized
plugin.initListener = listener
override fun onMBAudienceInitialized() {
//Plugin has been initialized correctly, can be started

Once you've done this ask MBurger to start plugins in your main activity (be aware that it has to be an AppCompatActivity), then, in app messages will be fetched automatically and showed, if they need to be showed.