So, following the recent update, the Plains of Eidolon was added. Currently, many players are have trouble/confused about how to get to it, as it shows up as locked on many people's star charts. Here is what you do:
first, go to Cetus on Earth, it should be right below the PoE Node. Once you have arrived, follow the path until you have reached the marketplace/bazzar. At this point, walk through it, and towards the back. You should soon see a giant orokin styled door, go into it, and you will be inside a hall, go out the door on the other side, and you will reach the Plains.
Note: currently the only way to get back into the city is to walk/fly(archwing) back to the door, where when you enter the hall the game will save your progress. If you instead Abort Mission, the game WILL NOT save your progress!
Note: you must have done Vor's Prize Mission to have access to Cetus!
Note: to use an archwing on the Plains, you must build the necessary launching module in your Clan Dojo
My Skyrim, Fallout 4, Starbound and WoW + other game mods at MODDB: