Man this tutorial is terrible...not only did it not bother to refer back to loading Skyrim.esm, later on it says:
"Before we can do anything else, we need to create a space to work in. This is most commonly done by duplicating an existing cell. The cell you choose will bring over some data you may need to modify, but we'll deal with that later. For now, locate and right-click on the interior cell aaaMarkers (No indication of where to find it...it's in the "world" menu under "cells", and pick "interior" from the dropdown menu) and choose Duplicate Cell (Except there IS no "Duplicate Cell" option in the right click menu.) You should see a new cell appear in the Cell List just below it (With no "Duplicate Cell" option, nope!). Rename this new cell by selecting it and pressing F2. We'll call this dungeon LokirsTomb."
So given that even if you can find the cell, you can't make a copy of it, because the right-click option they tell you to use isn't there, it would be very bad to try to proceed any further, since you would have to overwrite an existing cell instead of making a copy to edit.
Whoever wrote this thing obviously assumes that the person doing the tutorial has the telepathic ability to locate things without being told where they are, use commands that aren't implemented, and other impossible feats.
I don't know what's happening there, because I've managed to use the "duplicate cell" function perfectly OK, I just tested to make sure I could duplicate that particular cell OK and I get the option when I right click that cell in the cell view pane, and it duplicates fine. The tutorial looks to be correct to me.
Edit to add: I've had a look at the tutorial - given what you say the tutorial didn't say to do and the issues you are experiencing, it looks like you started in the wrong place - go back to http://www.creationkit.com/Category:Getting_Started and follow the instructions from the very beginning, which includes the following "Double-click to select Skyrim.esm for load - indicated by an "X" in the box on the left - and select "OK", and then goes on to tell you to create a blank esp to work in - I wonder if maybe you haven't followed that part of the tutorial before starting to make your own changes?