It was a good function that HealthUnlocked introduced a few months ago. However a few people can't understand why it isn't working. In quite a few posts it is appearing as @nnnn in plain text.
What you need to do to get the link created is type the @ symbol then start typing the person's user name and you will see the options for relevant people as you type each character. Obviously as you type each character the options diminish until you are left with a few people and then ultimately one. However you MUST SELECT THE USER from the list displayed to initiate the HUL function (ie if you just type you get @PeterWh appearing as plain text and not the link functionality).