If double-clicking a slideshow button on your menu just plays the slideshow and doesn’t open the slideshow editor, the slideshow may have been created in iPhoto. Slideshows created in iPhoto are sent to iDVD as QuickTime movies, and you can’t edit them in iDVD.
If you want to make changes to an iPhoto slideshow that you added to your iDVD project, you must make the changes in iPhoto, send the slideshow to iDVD again, and replace the old slideshow with the new one.
In map view, you can easily tell whether slideshows have been created in iPhoto or iDVD. iPhoto slideshows have a movie icon (a movie scene clapper board) in the upper-left corner. iDVD slideshows have a slideshow icon (two slides).