What computer operating system is your Premiere Elements 13 running on? Have you updated 13 to the 13.1 Update using an opened project's Help Menu/Updates? If not, please do so.
Generalizing for now - specifics when more information from you
1. Publish+Share/Computer/AVCHD and one of the MP4 presets there
2. Publish+Share/Mobile Phones and Players/Apple iPod, iPad, and iPhone/ and one of the MP4 presets there.
Please review and consider, supply more information about your source media and your export goals beyond the .mp4 file extension, and we will fine tune/customize the reply.