These are the first line of the .dat file:
<?xml version="1.0"?>
<!DOCTYPE mess [
<!ELEMENT mess (machine+)>
<!ATTLIST mess build CDATA #IMPLIED>
<!ATTLIST mess debug (yes|no) "no">
<!ATTLIST mess mameconfig CDATA #REQUIRED>
<!ELEMENT machine (description, year?, manufacturer?, biosset*, rom*, disk*, device_ref*, sample*, chip*, display*, sound?, input?, dipswitch*, configuration*, adjuster*, driver?, device*, slot*, softwarelist*, ramoption*)>
<!ATTLIST machine name CDATA #REQUIRED>
<!ATTLIST machine sourcefile CDATA #IMPLIED>
<!ATTLIST machine isbios (yes|no) "no">
<!ATTLIST machine isdevice (yes|no) "no">
<!ATTLIST machine ismechanical (yes|no) "no">
<!ATTLIST machine runnable (yes|no) "yes">
<!ATTLIST machine cloneof CDATA #IMPLIED>
<!ATTLIST machine romof CDATA #IMPLIED>
<!ATTLIST machine sampleof CDATA #IMPLIED>
<!ELEMENT description (#PCDATA)>
<!ELEMENT year (#PCDATA)>
<!ELEMENT manufacturer (#PCDATA)>
Seems ok to me. I used the "Add DatFile..." button for MAME and MESS, as the "Create..." button expects an executable file, so I don't understand your last suggestion.