The forum.tag views are not currently available. You must first create them yourself and add a menu to manage them.

UPDATE:

    <record model="ir.ui.view" id="view_forum_tag_form">
      <field name="name">view_forum_tag_form</field>
      <field name="model">forum.tag</field>
      <field name="arch" type="xml">
        <form>
          <group string="Forum Tags">
          <field name="name"/>
          <field name="forum_id"/>
          <field name="posts_count"/>
          <field name="create_uid"/> 
          <field name="post_ids" widget="many2many"/>          
          </group>
        </form>
      </field>
    </record>

    <record model="ir.ui.view" id="view_forum_tag_tree">
      <field name="name">view_forum_tag_tree</field>
      <field name="model">forum.tag</field>
      <field name="arch" type="xml">
        <tree string="Forum Tags">
          <field name="name"/>
          <field name="forum_id"/>
          <field name="posts_count"/>
          <field name="create_uid"/> 
          <!--field name="post_ids"/-->          
        </tree>
      </field>
    </record>

    <record model="ir.actions.act_window" id="action_forum_tag">
      <field name="name">Forum Tags</field>
      <field name="res_model">forum.tag</field>
      <field name="view_type">form</field>
      <field name="view_mode">tree,form</field>
    </record>

    <menuitem
      name="Forum Tags"
      parent="website_forum.menu_website_forum"
      sequence="20"
      action="action_forum_tag"
      id="menu_forum_tag"/>