I downloaded Forum (v0.1.2) and then installed OK and ran db/build.
Forum showed up in menu and was able to see it etc.
BUT. when I went to login to admin straight after adding the forum module I got the following error:
FATAL ERROR: DATABASE ERROR: Couldn't run query: SELECT `Post`.*, `Post`.ID, if(`Post`.ClassName,`Post`.ClassName,'Post') AS RecordClassName FROM `Post` INNER JOIN `Post` AS PostList ON PostList.TopicID = `Post`.TopicID WHERE (`Post`.ForumID = 41 and `Post`.ParentID = 0 and `Post`.Status = 'Moderated') GROUP BY `Post`.ID ORDER BY max(PostList.Created) DESC LIMIT 30 | Invalid use of group function
At line 397 in /var/www/sites/tools/silverstripe/sapphire/core/model/Database.php