When you create an iDVD project that contains a movie with chapter markers, you may come across an issue where you can't select a transition to use for the "forward" button in a scene selection menu. This can happen if you're trying to do this directly in the project window. However, you will be able to add the transition if you switch the project to map view. Here's a bit more about what's going on here.
When you add a movie with chapter markers to a menu, iDVD automatically creates two buttons:
If you double-click the scene selection button to open the scene menu, iDVD displays buttons named after each chapter in your movie. The number of buttons the menu displays depends on its theme (a theme can display up to 12 buttons per menu). Each scene menu contains a "back" button to go to the previous menu.
If you have a lot of chapters, the scene menu may only display the first group along with a "forward" button to navigate to another scene menu. If you want to add a button transition to this forward button, you will not be able to do this directly in the project window. To add a transition, switch to map view. For more information on scene selection menus, click here.
This document will be updated as more information becomes available.