To import players into the game Create a new League and click the button to select an import file. You must point it at a CSV file.
If your import file has players assigned to teams and you want them to stay that way leave the "Place in Draft Pool" box unchecked. If you check that box the game dumps all of the players into the draft pool.
You can find a sample file at http://www.barcodegames.com/ImportPlayers.csv