@bo5ton ; try adding a semicolon after the username. I think that's a change since this was first implemented.
You actually had to type a number? That's not much use. So if you type "@", directly followed by my username, directly followed by a semicolon, it doesn't work for you?
EDIT: As @ScallioXTX ; said. ^
Yup, I got that mention alright
The other way is to use [MENTION]username[/MENTION], which has the same effect but without that horrible trailing semi-colon and without any question over whether it's going to work properly or not!