You have to redownload the torrents again (as though you didn't have them) and double click them to start redownloading. MAKE SURE that the files you try and download already exist in the folder and that the client will point to them. They should then check that they are there and start seeding again.
If they start downloading, they are not in the folder OR your client has been told to save to a different one.
Example.
You file for "area loop.mp3" is in C:/downloads.
Your client needs to have that folder set as it's download folder as default.
When you try and download it again, it will see the file and presume it's finished downloading, chack it and, if complete, move it to the downloaded section and start seeding it. It may give you a message that it will overwrite the file (it won't, don't worry as long as you haven't messed around with it) so just click "yes" or "ok" or whatever ;D
I presume all clients work the same in this aspect as I use Azureus.
I've had to do this on several occasions when I've moved files, exchanged drives etc and it's worked fine.