Try this to get your torrent seeding :
You aren't meant to download the video file again since you already have it.
That message just means that you have to redownload the .torrent file, which you have done.
Now what you need to do is:
Stop the torrent Right click and under 'Advanced' choose "Set download location" Browse to where the actual video is on your computer and save Again right click an choose "Force recheck"The torrent will do a data check to 100% and be ready for you to seed.