If you haven't learned how to open the Cow Level already then listen.
1. Obtain Wirt's Leg inside of Tristram (drops once u click Wirts body).
2. Buy a Tomb of Town Portal from Akara in act1.
3. Defeat the difficulty you are wanting to open the Cow Level on.
4. Put Wirt's Leg and Tomb of Town Portal in the Horodric Cube and hit " transmute ". ( will not work if you have killed the Cow King in a previous game )