After you follow Niobe in the museum --
-- you'll be thinking to yourself, "Great, I have another way to get down here!" Yes, you do. But if you try to go out that same way (by clicking on the shelves to the left of the kiln), you'll discover a different puzzle on this side of the door. Phooey!
You need two references here to solve this puzzle: Niobe's note from the trash can in the Workshop, and the rubbing from the green plates in the Underworld. Until you have those two items, use the door that leads to the Underworld get in and out.
As Nancy says, you need to enter the actors' first names (Xenia, Grigor, Niobe, and Thanos) using the Greek alphabet and in Greek alphabetical order. Read to the end of this hint for an explanation of how to do this.
The first thing you need to do is write the actors' names using the Greek alphabet. Use the Greek alphabet rubbing you made to figure this out.
Let's start with Grigor, a fairly simple name. G in English is gamma, which looks like an upside down L. R is rho, which looks like a P. I is iota, and O is omicron.
Thanos is also fairly easy, once you know what theta looks like and realize the name takes a short O (omicron) rather than a long O (omega).
Xenia is a little harder because X is xi, which doesn't look like X; it looks like three horizontal lines. Niobe also has some issues because the last letter is a long E (epsilon) rather than a short E (eta), so you need the letter that look like an H.
Here's what the names look like written in Greek letters:
Now you need to put them in alphabetical order. Check the alphabet to see where the four initial letters appear: xi, gamma, nu, and theta.
The order you want is --
-- gamma, theta, nu, xi, so that's Grigor, Thanos, Niobe, and Xenia.
Now all you have to do is click on the letters of those four names in that order. When you finish a name correctly, a quarter of the outside rim will click down and stay down. When you've done all four, the door opens, and you now have a faster way to get in and out of the Workshop.