PHP needs to be configured or recompiled with:

xslt support is on your server. You must have libxslt installed and PHP compiled with the --with-xsl option.

Also, have PHP installed and recompiled with --with-dom-xslt' '--with-dom-exslt'

If you don't know how to do it, ask your host if they can do it for you. Not all hosting packages can do this, so make sure it is available with your hosting package.