Are you creating your slideshow in version 11 of the Organizer? If so, it's fairly simple to port the file to Premiere Elements 11. You can then combine the slideshow with your movie and output a DVD. There is no need to create an MP4.
If, on the other hand, you have a movie you've created in version 11 and you want to create a slideshow and combine these movies in version 15 or another version of the program, things get a bit more complicated.
Can you clarify if all of your work will be done in the same version of the program?
Then it's easy.
Complete your movie in Premiere Elements. Then, in the Organizer slideshow creator, select the Output and then option to Send to/Premiere Elements. The program will port your slideshow over to your Premiere Elements timeline, starting it at the point of the CTI playhead.
Very many thanks, again!