There are, of course, 3 different "versions" of the Ace libraries. The code quoted above is in the Ace2 version. Most new addons use Ace3. I don't think Ace3 provides this (at least, my lazy ass couldn't find it in the Ace3 libraries), just Ace2.
Ah, that does appear to be the case. The easiest course in that case would be the addon mentioned earlier in this thread, I suppose.
As far as I know, the /in command is (also) included in Deadly Boss Mods.