unable to parse XML-why??

im workin on Ubuntu..
anyway I have xml document : sample.xml

<?xml version="1.0"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<article>
bla bla..
</article>

URL is valid and i have net connection

aftercommands :
$ export DB="/usr/share/sgml/docbook/docbook-xsl-1.51.1"
$ xsltproc -o sample.html $DB/html/docbook.xsl sample.xml

theres an error and warning: failed to load external entity "sample.xml"
unable to pharse sample.xml

Any ideas??
Thanx
Danijela