Look for a Sports Interactive folder in your My Documents folder. By default, that's where saved leagues are store this year. I think that's your question, but I am a bit confused; how did you back up your league if you don't know where it is? Backing up would mean copying the save league folder from My Documents to someplace else.
Edit: Maybe you are looking for the backup files that you made with the in-game backup function? If so, this post explains things a bit better, I think:
http://www.ootpdevelopments.com/boar....php?p=2146206