hold down the shift key while depressing the @ key. A list will appear of Insider names. Whilst still depressing the @ key, type the first letter of the name you wish to 'mention' without typing any spaces. keep typing the name, and when you see the name that you want to mention, click it. nuhusker. You'll know you did it right, because you will see the name in blue now, in your comment. Remember, no spaces.
Hope this helps.