I tried it out and dug into your extension. The error is because your XML files have comments between the <xml declaration at the top and the <root tag. If you take out any comments between those, the extension loads just fine. This is due to the FGUnity xml parser being much more strict than the one from FG Classic. Classic allowed a number of things which are really bad xml, and just ignored it. Unity doesnt allow them.
If you want to include comments, just do it right after the opening <root tag. so they would look like.
Code:
<?xml version="1.0" encoding="iso-8859-1"?>
<root>
<!--
comments
-->
<whatever code>