You can open .jtd files with open office but it requires an extension to convert.
.jtd files are made by JustSystems-Ichitaro, this program does not use xml as it does not use plain text, but they produce another application called Ichitaro Arc
This can handle xml and with a plug-in OpenOffice can open the jtd files
If you go to the company website, it will need to be translated unless you Japanese is good,
they have a free download which converts to xml (i think).
Some articles i read,(dated 2010) stated that it was only available to windows and linux, but
the wiki link states that they have had a mac version for sometime.
I could not find the mentioned OpenOffice extension
It will be there but my searches returned no extensions that matched.
I hope this helps in someway.