Yeah I noticed that phpbb3 has this feature built-in. There is already a user bridge that would allow you to use phpbb3 (3.0.10) but it would not be as integrated as phpbb2 is with RN at current. I haven't had time to use it too much yet, so if you have any questions about it you'll have to ask over at tricked out news.
http://trickedoutnews.com/download-file-116.html
Any sort of future built-in compatibility for phpbb3 within RN, would also likely only be a user bridge, as opposed the current phpbb2 integration. The current phpbb2 used in RN has been heavily modified to function as a module, and the same would have to be done to phpbb3; and unfortunately there isn't anyone willing to take on the task for phpbb3 for a variety of reasons. Most notably the amount of work involved, as each time phpbb issues a release you have to update your version accordingly. Much easier to maintain a user bridge as you can use the unmodified phpbb releases to update your forums.
For me personally I prefer to stick with an integrated phpbb2 as most anything phpbb3 does can be done with mods for phpbb2. Having the theme of your forums match the rest of the site is nice, and I don't really need many of the features included in phpbb3. Not to say phpbb3 isn't overall a better option if you were evaluating stand-alone forums, as it has many performance enhancements and other options I might find handy if I were using it.
I can say a phpbb3 user bridge is on the roadmap, but not sure when it will happen. For now you have something to try via tricked out news should you want to venture down that road.
As for why the mod isn't working, hard to say without looking at it. I'm pretty sure it could be made to function although it was written for 2.021 and RN uses 2.023.. There are also some differences in file paths in RN as anything in the root/includes directory was moved to root/modules/Forums/includes