The 6.0 release is quite old, perhaps the option you are looking for was not implemented? I think all options should be in the list. If it's a hidden option of somesort then you will be forced to use Editor command line or ACL script to toggle it: "set promptnodtd=yes" or whatever is required.
The recommended way to work with new doctypes is to install the DTD into your "custom" folder and register its PUBLIC doctype identifier(s) within your catalogs file.