Your torrent program shouldn't 'find' the file, you should tell it exactly where the file is that you made the torrent from. That is, when the program asks you where to save it, point it to the original file in whatever folder it is, even if you have moved it from it's original folder.

PS If Transmission doesn't let you specify which folder your file is in (and I'd be surprised if it didn't), I'd suggest you use another client eg, uTorrent for Mac. You should be able to upload and download from same folder

Try the following:
1/ Stop the torrent in Transmission (don't remove)
2/ Copy the movie into the download folder (if there is already a file with that name, delete it first)
3/ Run a recheck of the torrent. Now Transmission should find it and check to 100%. It might as well move it into the upload folder, if you have set Transmission to move completed downloads there.
4/ Restart the torrent

Usually there should be shown a window with the files and the download location when you add a torrent to transmission. You should change in that window the download target to the file location of the file you want to upload, e.g. your upload folder, so that transmission finds it there insted of looking in the default download folder.

Well, I have created a new folder / directory "Transmission Uploads" (where the mp4 is) - besides my "Transmission Downloads" folder. So upload and download should not interfere.
I did quite a lot of search in forums. Could it be, that my upload is not "approved" yet (after 4 or 5 hours)?
Sorry, I'm very new to this …

Thank you for your advice anyway!

Boris / vollbart71

After you downloaded your own torrent, did you then open it in your torrent client as you would any other torrent. When it asks where to save the file you point it to the file you made the torrent from. Your torrent client might say it will overwrite file, it won't. It will check that the file is the same and then start seeding