ArcMap is the map editor used to create the worlds of Arcadia Online. You can download the editor and play with it to create a unique map. If your map is interesting enough you can submit it trough the forums and it will be used when a new realm goes online!
For now the only OS supported is Windows. If you are a developer interested in helping to make ArcMap better, implement support for another OS or you are simply interested for the code behind ArcMap contact us and we will be happy to help you!