Bit of a nuisance, really. Why wasn't this content included in the DVD installation discs in the first place? (Rhetorical question, NNTR--no need to respond). I have done this twice now, I'm fed up and taking my material to someone using a different DVD creation application and paying them to do it.
1. UNINSTALL ENCORE. Including preferences if you feel like it, although its not necessary.
2. REINSTALL ENCORE
3. Open the file noted in Ann Bens post above and drag each of the folders to REPLACE the content within the enclosing Encore application folder. If you are worried about this, first open one of the existing Library folders that was installed with the application and compare its contents with the contents of the same-named folder from the download. You'll see the same files, but dozens more.
Finally! We now have the missing content WITH buttons in the application.
By the way I know that uninstalling isn't necessary in most cases. The reason I did it was that I'd attempted a few times to replace those folders and still wasn't getting the buttons. What I discovered, and the reason why I decided to uninstall/reinstall, was that there was a greyed out "second set" of folders which was probably from one of the attempts to replace the original ones. So I'm only suggesting the above route if you've experienced similar frustration.