Username Auto-Completion (Mac & iOS)

How do I get Twitterrific to auto-complete usernames when I write an @ Reply or Direct Message?

For an @ Reply, type the "@" symbol in the compose window, then begin typing either the username or the real name of the person you want to mention. Twitterrific will display a list of matching names for you to choose from. For a Direct Message, type the letter "d" and then a space, then begin typing the username or real name to see the list of matching names.