I can't import old projects from previous old versions?

[Dr Brent H15D5]

Which extention do I import? .macc or .prv or .prel ?  I have so many of these files.

[Bill Sprague]

You can't import anything but actual media files of video, photos and music.   You may be able to open old projects.  

.prel is the project file extension.  But, NOT built into Premiere Elements is any promise of backward compatibility.  Depending on what 'old' features you used, the newer version may not be able to handle them.   And, worse, can be that the old project is 'corrupted' by being opened in a newer version.   All you can do is make a copy of the old project and try to open it in a new version.   It may or may not work fine.  Projects with no effects or transitions have the better chance.  Some versions of PrE had unique background music systems.  Those can cause trouble too.

[HOTELECHOMIKE]

But, NOT built into Premiere Elements is any promise of backward compatibility.

I think you mean no forward compatibility.  Anyhow, neither forward nor backward compatibility seem to be certain.