Vue2

Global Register

import Vue from "vue"
import DropdownMenu from "v-dropdown-menu/vue2"
import 'v-dropdown-menu/vue2/css'

Vue.use(DropdownMenu);

Local Register

import DropdownMenu from "v-dropdown-menu/vue2"
import 'v-dropdown-menu/vue2/css'

export default {
  components: {
    DropdownMenu
  }
}

Via CDN

<script src="https://unpkg.com/vue@2"></script>
<script src="https://unpkg.com/v-dropdown-menu/vue2"></script>
<link rel="stylesheet" href="https://unpkg.com/v-dropdown-menu/dist/vue2/v-dropdown-menu.css">

<script>
new  Vue({
  el: "#app"
});

Vue.use(DropdownMenu);
</script>