You type @ before their name, with no spaces. Once you start typing, a drop-down box will appear with matches, and you can simply click on the person's name.
There are a majority of people on the interwebs who cannot tag in their statuses, but the majority who can tag their friends by typing the symbol @ before typing a friend's name. For example: To tag...
Well if the person has a facebook you would say "Last night i went to the mall with @joe." the @ sign will go away when you press share. But if they dont have a facebook you would just say, "Last...