CentOS 6 is not supported at this point. Please take a look at this ticket: https://issues.jboss.org/browse/BGBUILD-267 . Main issue is that grub uses different directory structure compared to previous CentOS versions which makes appliance-creator uncappy. We reported this here: http://bugs.centos.org/view.php?id=4995
Our goal is to support CentOS 6 before 0.10, but I cannot tell you when it happens exactly.