The Kodak EasyShare Z990 appears to use a 48.5mm front thread on the lens barrel. That is not a standard photographic filter size, so finding a screw-in filter made specifically for it may be difficult.

Based on the community response, some users have reportedly adapted 49mm filters by slightly modifying the threads, but that is a DIY approach and may risk damaging the filter or camera threads.

A filter adapter tube is another possible solution, but on a superzoom camera like this it can cause vignetting or darkened corners, especially at the wide-angle end.

So in short: the thread is about 48.5mm, which is non-standard, and using standard filters will likely require either modification or an adapter with possible drawbacks.