Premiere Elements (any version) does not support MXF files.
You would need to convert them to another format compatible with Premiere Elements supported formats. Have you done that?
What computer operating system is involved?
There are numerous MXF converters to be found in a google search. I do not work with MXF so I do not have one readily at hand to share with you.
But, there are numerous choices for a MXF converter to be found online
Please review, consider, and then let us know of your progress as your schedule permits.
Add On...Have you updated 13 to 13.1 yet using an opened project's Help Menu/Updates? If not, please consider doing that.