For any custom league files, you can find the location to save them by going to tools > edit default league > open folder.
You should see a folder in windows explorer called "defaultleagues". Inside that folder you should see other folders like 2010,2011,2012, etc. Make a D3 folder and place the files from the D3 mod there. You should have conf.csv, teams.csv, and tourneys.csv. Then you you start a new game, D3 will be one of the options in the League File dropdown box.
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