Leopard comes with a brand new version of Interface Builder. The classic palette system is gone, replaced by a new plugin model that renders the old F-Script palette unusable.
In order to fix that, I have implemented a plugin for Interface Builder 3. It contains two elements that you can drag-and-drop from the Library window of IB (shown below) into your interface:
There is more about embedding F-Script into your own applications in this article.