A .prel file is a project file. It is not media.
You open it using File/Open.
It appears you're trying to add it to your project using Add Media.
Also, are you saying that you're trying to open a version 3 project file with version 11?
I would very much recommend against that. The program has changed pretty drastically in the last several years, and, even if you can open it, it's liable to lead to some severe corruption of the project.
Thank you for your response. I'm just trying to open my project. When it wouldn't open in PE3 I paid for the PE11 download. It won't open there either.
Since you last Opened that PrE 3 Project in PrE 3, have you changed computers, changed OS, replaced the system HDD, anything like that?
With such changes, it could be that a necessary CODEC, for some of the media in that Project, is no longer in the OS, or available to PrE 3.
As most CODEC's are installed into the OS, that could contribute to PrE 11 not having the correct CODEC either.
Do you know which media files were used in that Project? Are they still on your system?
If so, you can use MediaInfo, or G-Spot (both free, and very good), to determine what the CODEC is, in those files, and whether you still have it installed on your computer. If you will post the info from MediaInfo, or G-Spot, someone here can likely direct you to the necessary CODEC.
As Steve points out, Opening a PrE 3 Project in PrE 11, will not likely yield a positive editing experience, though if the Project is very simple, it might work.
Please let us know a bit more about the CODEC inside the media files, and good luck,