Are you using the mobile app or the mobile view in a web browser? Also, tag someone where? In a post/reply?
If you are talking about replies to a post, you cannot. If you are talking about the original post of a thread (either on an article, blog, Discussions, or message wall), there should be a comment form at the bottom of the thread. Note that your account is not yet autoconfirmed. Some wikis require accounts to be autoconfirmed before they can post/edit. An account become autoconfirmed 96 hours after it has been registered. There is also a requirement to confirm you email but it looks like you already did that.