I have taken Amtrak to Milwaukee and the bus 90 to Miller Park drops you off 100 yards from ballpark.
I drove to a game this year ----- never never again. Traffic is a mess all around Milwaukee and getting out of the Miller park lot is like watching paint dry. Two hours to get out and on to the interstate for a night game and day game.
Take the bus as it has a special entrance/exit to Miller park. Check Brwers web site for more info on transit.
For public transportation, the Milwaukee County Transit System (MCTS) using Route 90 from Wisconsin Avenue downtown. For more information, please call the Bus Line at (414) 344-6711 or visit www.ridemcts.com.