Well, if you want your reply to be noticed by the one conversing with you, post it on their wall, not yours.
Not entirely sure what you mean by the "mention tag," unless you mean prefacing a username in some other post with the "@" sign.
I haven't been on forums for a while. All these updates are confusing :P Plus I went with the mention tag, typed in a name and it went all crazy. Tried to fix it and ended up the same. If its not intuitive it ain't worth knowing xD