Weaver Xtreme has significant changes to the <div>s for various reasons that make it much more flexible than Weaver II. Premain is gone because there is no #main any more, so it doesn’t make sense to have it. It is not even possible to add it back. But the identical functionality is still there.
But as @thesurge says, post header (for full width right after the header), or Container Top (top of container, but includes container’s margins) will serve the same purpose.
As with Weaver II Pro, these particular insertion areas are just in the premium Plus plugin.
See http://demo.weavertheme.com/insertion-areas/ for a complete demo / examples of all insertions areas.