I am trying to import an xml file but I get a parsing error.

Do I need an XSl stylesheet?

Does the file need an .xml extension?

Yes you need an xsl stylesheet, and probably yes you need an .xml extension (never tried it without one; can't imagine why you'd use any other extension). You'd need an .xsl extension on the stylesheet.

What's the best way to create an XSL stylesheet for FMPXMLResult grammar?

Take a look at some of the examples here. There's quite a few.

Look at some of the (simple) examples at FileMaker's XML Library page.

http://www.filemaker.com/products/technologies/xslt_library.html

Get Beverly Voth's book.

http://www.amazon.com/FileMaker-Developers-Wordware-Library-Filemaker/dp/155622043X

And/or post your xml here (if it's short), within code tags ("code" and "/code" within [] brackets; I can't type it here).

And/or (preferably) attached to your post as a zipped xml file (which is actually "text" with a ".xml" extension). That way we don't have to muck around fixing the minor damage which the php parser here will do to unprotected xml code.

Basically an xsl stylesheet has to be custom-built for the xml data. XML means "extensible markup language," so it's whatever the author decided, within the (strict) rules of xml. Much like a relational (multi-table) FileMaker file. That being said, there's a lot of similarity between simple xml files, therefore between their xsl stylesheets.