You type @ followed by the username, which is sometimes different than the display name. For anyone with a different display name, you can see the username by hovering over the display name.
Or, even easier, in the full reply screen, click on the Insert User Link button (which has an @ next to a faceless person in a blue shirt) on the top right side, just over from the smileys button. Then you can type in the name as you know it, and it will give suggestions. Like, if I want to tag our favorite art historian, I can just type @ lp521 (without the space) or click on the box and start typing ProfessorArtNerd and click on her name when it appears, and it will do the work for me: ProfessorArtNerd.
(I used that example because I know her two names are different!)