What version of Premiere Elements and on what computer operating system is it running?
Generalizations for now....
The project file for the program is a .prel file.
If you want an video export with a .mp4 file extension, you can use
with one of the .mp4 presets
Mobile Phones and Players
and one of the .mp4 presets for the Apple iPod, iPad, or iPhone to get a H.264.mp4 file.
To get details specific to your situation, please supply details for us to help you.
Any questions or need clarifications, please do not hesitate to ask.