I'm not sure what your problem is, and maybe someone will pop in that has a clue, but until then here's some ideas.
Since you're posting on this thread I'm assuming you've read the first post, right? You might want to double check the sections "Exporting Your World", "Installing Your World", and "Uploading Your World" just to make sure you didn't miss a step.
Did you "rebuild routing data" before you saved and exported? If not, that
might be the problem. Whenever you're going to export always make sure to go to View, the "Rebuild routing data" (it's at the bottom). SAVE, then export.
As for the "already installed" issue, read the "Installing your world" section in this FAQ and see if that helps.
I hope some of this helps, oh and if you haven't already, you might want to check out
this video about exporting worlds, as that might help you out too, especially if you're a visual learner.
I hope some of this helps! Let me know how it goes. ^^
