We are aware of some issues relating to the searching of forum posts that we are trying to fix. First of all, apparently the search function doesn't work on any topics or posts that were migrated to the site from the old Ning site. Secondly, the default search is for "Forum Topics" instead of "Forum Posts," so, unless the word you are searching for actually appears in a Topic Title that was posted post-import, then it won't find anything. When you get to the 2nd search screen, you can then select "Forum Posts" from the drop-down menu, but again, those searches will only give you results of post-import discussions.
That, and the fact that you have to select and delete "Search Forum" from the search bar really drives me crazy. We'll try to fix that, too.
Let us know if you notice any other bugs or issues in regards to searching the Forum (or the site in general). Thanks!