I too was frustrated not to find the KTEL schedule on line...
So far, all I know is that the 845 or the 871 bus both seem to go from Piraeus to Elefsina. Only problem is, I don't know 1. where they stop in Piraeus; 2. How to get to the stops from the port; 3. How often they run to & from Eleusis; 4. Where they stop in Eleusis in relation to the museum & archaeological site (I'm walking) - I can't even find the Eleusis bus station (if there is one) in google maps!!!