"Wilhelm Scream" is a generic foley term for... well... the scream. The term may be either trademarked or copyrighted... and it isn't used on any of the screams in Apple Loops... more on this... The sound effect itself is not in the public domain.
There are, however, many screams in Apple Loops.
In /Library/Audio/Apple Loops/ — (I've reorganized my loops so the order isn't the same as the original installations), but what you are looking for is a folder labelled "Jeremy Holland". Inside Jeremy Holland there is Sound Effects and you want the "People" folder.
This folder is FULL of "Wilhelm Screams" - almost 90 of them. They are, amusingly, called ****helm Scream. For example Aaronhelm Scream, Bradhelm Scream, Coreyhelm Scream (I swear I can tell this is Corey Feldman's voice!), etc...
There are others, not quite Wilhelm Scream effects, but interesting. In the Chris Moulios folder there is "Lower and screamy" which sounds a bit like Hypnotoad (Futurama), and Detroit Chop Shop has a few interesting "scream" effects that are completely synthetic... or synthesized (in their Motions & Transitions folder).
It's a fairly comprehensive collection of "Wilhelm Screams"...but not one of them is called... Wilhelm. :D