No idea what Sigil is either, but I use Calibre to convert my files. Free to download, easy to use. Just hit add books, browse to find the file, highlight it, hit convert, choose the format you want to convert to from the drop down menu on the right hand side, hit go and a couple of minutes later, there's your file.
LabRat
(who needs these things to be so simple a wombat with a lobotomy could do it.)
