What you have to understand is that there are several different slots that levels can occupy. By default, the game will first go to MAP01, which under normal circumstances is Greenflower Zone Act 1. Many addon WADs replace the first slot, so that you naturally go directly to them. A few others change the slot that the game starts out in. And there are some, like yours, that put the map in a slot other than MAP01 but don't set the game to go to their slot.
To play that level, you have to figure out which slot that level took. You can figure that out in the console...after adding in the WAD, it should show the map number or numbers that have been filled. Let's say that a WAD filled in the slot MAP03. You can go there by typing "MAP MAP03" in the console.