When you configured BitComet, it set up a default download directory. That is where it will place downloaded content. You can change it in your preferences at any time, though that will only affect future downloads, not those already completed or currently in progress. So check your preferences to see where your default download directory is, and look there for your files.
Don’t move or delete them from there until you’ve finished seeding, or BitComet will stop and complain that it can’t find them anymore.