1]Look, as far as I know, if you are playing only Diablo 2,[without expansion], you need to kill diablo.[Last level].
2]Then, "in the same difficulty", go back to rogue's camp via waypoints.
3]Then, put the tome of town portal and the wirt's leg in the horadric cube, click transmute.A red portal will open in the camp from where you can access the cow level.
In case you have LOD installed, and you are playing the "expansion character", then you have to kill baal.[complete all 5 acts].Then do the same thing mentioned above,[steps nos. 2 & 3].
Everything else related to cow level is mentioned in the previous replies.It works in single player.I don't play multiplayer and hence have not tested it yet.
I hope this is VERY CLEAR now.