Xenia's tablet is the black square at the left of the table. When you pick it up, you see only six boxes with locks on them and one with a question mark. Click on the question mark to find --
-- a sort of jigsaw puzzle. Read to the end of this hint for a picture of the completed puzzle.
The pieces are on the right; left-click to pick one up and put it in place. They're all right-side up and don't have to be rotated. When a piece is put in the right place, it clicks and can't be moved again.
If you don't see any pieces you can place, use your mouse's wheel to scroll through the list and find one you recognize. I usually start with the top border, which is quite recognizable.
The completed picture, which is four women in long robes looking decorative, looks like this: