The new multi-tab SideBar is an experiment and is thus currently only available to wikis set up as "Educational." Tweaking will be done based on the response to the changes and then it will see wide release at some point in the near future. It's not quite clear when that will be currently.
A wiki can be set to "Educational" when it is first created. PBwiki asks you "What will you be using this wiki for?" with a list of various functions.
There is currently no way to change your wiki's "function."
@@@@@