Trouble with Submitting to Smashwords

Hi everyone,

I'm trying to use Calibre to add a cover page to my ePub novel. It seems to work fine but when I upload the completed file to Smashwords I get the following message...

File 'META-INF/calibre_bookmarks.txt' in EPUB not listed in manifest!

I also get an email from Smashwords saying the following too...

element "nav" not allowed anywhere; expected the element end-tag, text or element "a", "abbr", "acronym", "address", "applet", "b", "bdo", "big", "blockquote", "br", "cite", "code", "del", "dfn", "div", "dl", "em", "h1", "h2", "h3", "h4", "h5", "h6", "hr", "i", "iframe", "img", "ins", "kbd", "map", "noscript", "ns:svg", "object", "ol", "p", "pre", "q", "samp", "script", "small", "span", "strong", "sub", "sup", "table", "tt", "ul" or "var" (with xmlns:ns="http://www.w3.org/2000/svg") (repeats 2 times)

I've looked online and seen a few remedies but I can't follow them, too complicated for me! Can anyone explain in really simple steps how to use Calibre to overcome these issues so that Smashwords can accept my book?

thanks

Dev