$('select.dropdown')
  .dropdown('set selected', ['meteor', 'ember']);

<select name="skills" multiple="" class="ui fluid dropdown">
  <option value="">Skills</option>
  <option value="angular">Angular</option>
  <option value="css">CSS</option>
  <option value="ember">Ember</option>
  <option value="html">HTML</option>
  <option value="javascript">Javascript</option>
  <option value="meteor">Meteor</option>
  <option value="node">NodeJS</option>
</select>