that's funny - and you're quite sure you're standing BEHIND the guy you're attacking, correct?... the very first time I tried, I was standing at the enemy's SIDE, and that doesn't work. Also you need to get up RIGHT up close behind their back, not a few feet away. Disregard if you've already tried this (I don't want to insult your intelligence) but I myself forgot these things when I first tried to backstab so I thought I'd mention the bleeding obvious anyway
You don't have to use any special button to actually do the backstab, just use the regular attack button.
Can you backstab with ANY weapon at all? If not, then something's not standard procedure here, who knows, it might be a bug...