Thank you for your post.
Unfortunately, FileMaker Pro 8.5 is the earliest version of FileMaker Pro that is certified to run on Leopard (10.5.x). However, I do have a copy of FileMaker Pro 6 on my machine to test out old files, and I'm able to perform a find without a problem.
If you have access to another computer, install FileMaker Pro 7 on that computer, create a simple database file and execute a find. If this works, then there may be something wrong with your file.
On your original machine, is the Find problem occurring with a specific file or all files? If just a specific file, then I would run the Recover command (File menu) and when finished, open the recovered file and try Find again.
Please keep me posted to your progress.