.. What do I do with these green plates with partial letters?

There are four of these plates in the Underworld. For now, just notice where they are.

If you could put them all together, they'd show you the Greek alphabet and its equivalent letters in the English alphabet. You'll need this before you can solve the Famous Greeks puzzle.

What do I do with these letters and symbols in a circle?

What you're after here is a rubbing, like Nancy did in ND 13, Last Train to Blue Moon Canyon, and other games. So you need a piece of paper and something like a crayon or a piece of charcoal.

The paper you can get from the printer in the museum office.

You can get a piece of charcoal from the Workshop.

The Workshop

Once you have both, use the paper and charcoal on all four green plates. There's one in each of the four sections of the Underworld; see the map if you're missing one.

Map of the Underworld