In order to find the secret cow level you are going to need a few things.

1.Wirts Leg
2.Beaten the Game
3.Tome of Town Portal (with scrolls)
4.The Horadric Cube

Once you have beaten the game go to the Rogue Encampment in Act I, then put Wirts Leg and a Tome of Town Portal (This has worked with random ammounts of scrolls in the tome) into The Horadric Cube. Once you hit transmute they should disappear, but once you close your inventory window there is a red portal that will lead you to the Secret Cow Level. This ONLY works in the Rogue Encampment in Act I.