What version of Premiere Elements are you working with and on what computer operating system is it running? What version of PowerPoint do you have? Background information for now.
A prel file is the project file for a Premiere Elements project. It is not a video file that can be imported into non supported software.
You cannot even open a Premiere Elements project prel in another project.prel.
You can create a Timeline in a Premiere Elements project and export that Timeline to a file saved to a format that is compatible with PowerPoint.
And, then you Insert/Video that file into your PowerPoint presentation. With Microsoft PowerPoint 2010, this seems to be what is supported video-wise
Please review, and then let us know if you need clarification on any of the above.