Where is the Persistence tool window in Intellij IDEA 15?

If you haven't included JavaEE Persistance when you created your project as @Freek de Bruijn suggested, you can go into:

File - > Project Structure - > Modules -> + -> "JPA"

That will enable persistence under Tool windows as well if, like me, you checked out your project from version control. Took me quite a bit of time to find that setting actually.

The persistence support is available in IntelliJ IDEA Ultimate Edition (and not in the Community Edition, I think). When you create a new project in for example IntelliJ IDEA 15.0.3 EAP (build 143.1453.5), the New Project dialog opens:

There you can for example select "Java Enterprise" in the left panel and "JavaEE Persistence (2.0)" in the "Additional Libraries and Frameworks" list. Now the Persistence panel can be opened via the View | Tool Windows | Persistence menu. The Persistence panel is visible here in the bottom left part:

Echoing what @0x6C38 said here. File-> Project Structure -> Modules -> + then type in JPA.

Then you will have it in the Tool Windows.