We've had a number of questions recently about searching in Mac OS X 10.5.6.
As we explain in this knowledge base article, Apple introduced a bug in Mac OS 10.5.6 that causes it not to correctly send search requests.
Group Logic has been working with Apple and encouraging them to fix the problem in 10.5.7. We'll post an update as soon as we confirm that a fix is available.
Until Apple releases a fix, there are a few workarounds. One is to simply stay with 10.5.5 for now, if feasible. Another is to use a free tool called Find Any File, as described in this knowledge base article.