Traveling to the Secret Cow Level
To access the Secret Cow level, you must first kill Baal. After you have killed Baal (making sure the quest has been completed), you can access the Secret Cow Level for that difficulty level.
Go to the Rogue Encampment in Act I. Place one Tome of Town Portal + Wirt's Leg into the Horadric Cube and hit the Transmute button. A Red Portal will now open to the Secret Cow Level. You must transmute this combination in the Rogue Encampment for the portal to open. If you hear a message, "I can't do that" then you are doing something wrong. Make sure you have completed the last quest in Act V, that you have placed the correct items in the Horadric Cube, and that you are transmuting the items in the correct location.
Wirt's Leg can be secured in Tristram. Wirt's body is located in the top left quadrant of the town. Click on Wirt's body and, amid a shower of gold, his Leg will drop on the ground. Don't worry if you have been there before and forgot to get the Leg -- you can always go back and get another one by returning to the Stony Field and entering the Red Portal into Tristram.
Once you have Wirt's Leg, return to the Rogue Encampment and put it and a Tome of Town Portal into your Horadric Cube. The Tome does not need to be filled to work, so if you do not want to use the Tome you have filled with 20 Town Portals, you can simply buy an almost empty one from Akara.