There are a total of 20 areas within the MMO map. A few marked below might not be needed for the achievement to unlock.
The easy ones that are listed on the map are:
Old Goat Mountain
The locations that weren't on the map are:
Twistram Cathedral - *highly miss-able!* on the west side of Twistram is a path that leads you up to the church. As soon as you walk in, the location changes and is counted.
Throne of Goatson - you have to complete the quest from DumbleDoor The Grey at Twistram. This will get you the "Not Again" achievement, as well as stuck out in space.
Your House - fairly missable, from the Adventurer's Path, the first building with an open door you see on the second floor is your room at Your House. You have to walk into that room for the location to change.
Launchpad - follow the path behind the lady that wants you to destroy the beehives up to the lake, then head towards the girl resting by a tree. Behind her and the tree is a hole that you must jump into. (This location might not be required for this to unlock, but will get you the "Music Non Stop" achievement.)
Server Room - entering Alvesta, head to the treehouses on the left, and jump out the window onto the landing beside the higher treehouse. From there, go up the walkway beside the STOP sign, where a panel seems to be open. (This location might not be required for this to unlock, but you need to go there to trash the room for the "Voluntary QA" achievement.)
Adventurer's Path - can't really miss, it's the pathway from Snowflake to Gold Farm
Goatwind Wall - can't miss while going to the portal for Goatwind
Old Goat Pass - can't really miss, the pathway connecting the beach and the mountain
If you have any questions on these locations, you can check out the Wiki page that lists the locations - http://goatsimulator.gamepedia.com/Category:Goat_MMO_Simulat...