This question has been asked here several times.
Older MindManager files use the mmp extension
Newer use the mmap extension.
If you can find a MindManager vs 14 download, a trial version will do, you can open the mmp files and save as mmap.
NOTE you will have to find the vs 14 download yourself since it is not available from the normal download area anymore.
I've a installation setup for MM14. Write me a note to email@example.com and I'll send you the .exe via wetransfer.com
It also still seems to be available via Wayback (the Internet Archive), along with several other older versions. I managed to download it, though it was fairly slow: http://web.archive.org/web/20150206130522/http://mindjetdownload.s3.amazonaws.com/MindManager_14.3.192_EN.exe
I haven't tried to install it yet, but it looks to be intact.
Actually there is still an “official” link on the MindManager website, but it’s not on the downloads page. There is an article in the MindManager Knowledge Base on how to do the conversion, complete with a link to the MM14 download: https://kb.mindmanager.com/125326