See the "Quick Start" section of the following page:
http://www.openfeint.com/ofdeveloper/index.php/kb/article/000028
You add a leaderboard to your game using OpenFeint's Developer Portal. That gives you a leaderboard ID. You might have more than one, such as different leaderboards for different game modes.
I'm almost certain that's the ID that OpenFeint.SubmitHighScore is expecting. I say "almost" because I don't have the SDK with me to check. But there is a native Cocoa example on the page I linked above which backs me up:
Note that no user ID is needed for this call, so it's a safe bet that the Unity wrapper works the same way.
You also asked about delegates. Delegates are methods that you pass in as arguments. (You can think of them as function pointers, if that helps.) In the setHighScore method above, the onSuccess delegate would get called if the high score was successfully set, onFailure would be called if the operation failed. (However, this non-Unity example appears to be passing in empty delegates, so no action will be taken in either case.)