To add transitions to buttons:
Select a button (click it once) on the menu to which you want to add a transition.
To select more than one button, press the Shift key as you select buttons. To select all buttons, first select one button, and then choose Edit > Select All Buttons. The transition you choose applies to all selected buttons.
Press Command-I to open the Button Info window.
Choose a transition from the Transition pop-up menu.
If the second pop-up menu changes to say “Right to Left” (shown below), you can choose the direction the transition plays. To choose a different direction, make a new selection from the pop-up menu.
To see what the transition looks like, click the Preview button (shown below).
This opens the iDVD remote control. Using your pointer to work the arrow buttons on the remote control, click the button or buttons on the menu to which you added a transition. Click Exit on the remote control to return to the menu.