IIRC, the steps don't appear at the chasm that blocks your way to Level 27. They can be found in the Northwest corner of the cavern, or to your right as you enter the area from the stairs up. These lead to a series of teleports through the inner structures, which are part of the "Long Walk" at the end of the game.
You need three sets of Power Crystals to "power" up Level 26, so you'll be coming back after you find them. Let me know if you still can't find the plates.
[the tatter 16-10-2011 16:24:37]:
I am on level 26, and have shot all the mouths, and got the message that the way was open, but the flamming steps did not appear. I have looked for a hidden switch, or a place to put the jewel, but could not find anything.