Apple hasn't enabled Ringtones yet and so, no Ringtones category. You'll have to wait a bit more.I have no tab under the Library, (is the function live yet) which is show on Apple's itunes web site.
I have the tab when I select my iPhone, to upload them. But without the tab in the library I have not way to create them. Nothign to upload.
Mac OSX 10.4.10/ iTunes 7.4.1 (2)