There are two different secret levels. First, there is the secret level. The entrance for this level is in level 15. Once in the secret level, there is a secret exit off of that level to the Super Secret Level.
To get to the secret level:
1. When you begin go down the lift to the left. Then run to the large pit and jump in.
2. Look for a teleporter to the right side of the middle building. Step on it.
3. You should be facing a BFG9000. Get it and fight the Baron that comes out of the wall.
4. After the Baron is dead the wall will back to where it came from. A Pain Elemental will come out of the wall. Kill it too (Duh).
5. When the Pain Elemental is dead get the Yellow keycard. Turn around, fight some guys, then hit the button that came up from the floor. This will raise steps. Go up them.
6. Kill the guys up there then get to the place where you are north of a raised platform of the pit. This platform should have a radiation suit, energy cell, and a button on it.
7. Jump onto it. It is easier to use the map because you can`t see it from where you have to jump. Get the stuff and hit the button. That will open a door on the left wall in the pit. Go in there.
8. While in there walk around till you find an invunerability sphere guarded by some Cacodemons. Take the sphere and kill the Cacodemons. This will open a door in a hallway near to where you get the blue keycard. Go to the first teleporter you went in and go up the steps and jump towards the castle like building. Go through the yellow locked door. Then you should see some platforms with a texture going down the middle of the floor like the one in the pit. Go straight.
9. In there you should find and open hallway with a funky flaming texture on the walls. Go in there and follow it to a transporter. This is the exit to the secret level. Go in!
Location of Exit to the super secret level from the secret level:
 
Use IDDT to see where this is, or play the level and look at the map. I didn't think that a walk-through was required for this one.
