Step 2. API usage

First, you have to get APIManager.

QuestAdderAPI.getInstance().getAPIManager()

Second, just use it!
There is my code example.

        QuestAdderAPI.getInstance().apiManager.addItemDatabase(object : ItemDatabase {
            override fun getKeys(): Collection<String> {
                return ResourcePackManager.getItemKeys()
            }

            override fun getItems(): Collection<ItemPair> {
                return ResourcePackManager.getAllItems().map {
                    ItemPair(it.first,it.second)
                }
            }

            override fun reload() {
            }

            override fun requiredPlugin(): String {
                return "Fantasia"
            }

            override fun getItem(p0: String): ItemStack? {
                return ResourcePackManager.getItemStack(p0)
            }
        })
        object : Listener {
            @EventHandler
            fun apply(e: GiveRewardEvent) {
                Fantasia.getPlayerData(e.player)?.let {
                    UpdateExpEvent(e.player,it,e.exp.toLong()).call()
                }
            }
        }.register(fantasia)
        QuestAdderAPI.getInstance().apiManager.addAction("setclass", ActSetClass::class.java)
        QuestAdderAPI.getInstance().apiManager.addEvent("skillaction", EventSkillAction::class.java)