Wow, thanks for digging up a really old thread! It's about a version of Numbers that is radically different from the current version, a completely different application in all but name.
But it is still the case that you cannot link via formula from one Numbers document to another (you can transfer data by script, though that is seldom more efficient than good old copy-paste).
Many think it is a good thing that you cannot refer in a formula to another document. Links between documents are easily broken when something is moved in the file system. And how would they work across platform, say on iOS, where there may be more Numbers users than on the Mac?
Numbers, unlike Excel, makes it easy to put more than one table on the same sheet. And you can have multiple sheets in the same document. So for most projects you've got plenty of ways to organize your work without having to resort to links between documents.
SG