We just took it last week. Thanks to all the detailed posts as well as @VMax1700's photos we were able to find the stop and ticket kiosk. The more complicating detail for us was that we were docked at Terminal C which I had not previously read about. It is probably the most convenient terminal because it's right by the entrance, but because we were looking for the X80 stop it took a bit of running around. Terminal B is next to Terminal C. Theoretically we could have been stopped by the security going into the Terminal B area- my mom who was following us but did not plan on going to Athens was and told us about it later. We went through the parking lot and showed a picture of the kiosk to someone at the terminal who pointed in the direction of the bus stop. The bus stop is straight ahead from the main entrance of Terminal B, not to the left where the parking lot is and which leads to the gates. The bus was running about every 30min so even though we got there at 8.10 am, somehow we did not leave until closer to 9. We got off at the Acropolis with no problems and the driver pointed us in the direction to walk.
On the way back, we had planned on looking for the stop at Syntagma square. However due to the heat, decided to use the same transit pass to get on the metro from Monastriaki to Piraeus and then the bus back to the terminal. We've done the walk before from the train station back to port on a previous trip, but had forgotten how long the walk was until we took the bus back this time.