@joankung
Hi,

You could add new tag in the Edit task page. In the Edit task page, click on " Edit more " and select "Tag", then click on "New Tag" in the tag list to enter a new tag, click "OK " to save. See the pic: http://faq.doit.im/android_4.html#59

Could you please which step you have problem?

Best regards,
Doit.im Team